Remove FilterExpression from Get-AzVMImage documentation

[Sandido]

As per the PS design request, https://github.com/Azure/azure-powershell-cmdlet-review-pr/issues/563 , the FilterExpression parameter was never added and should be removed.

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[msJinLei]

Please fix the failed test case

2021-01-14T06:34:39.5411810Z   [xUnit.net 00:02:52.23]     Microsoft.Azure.Commands.Compute.Test.ScenarioTests.VirtualMachineTests.TestVMImageCmdletOutputFormat [FAIL]
2021-01-14T06:34:41.6506855Z     X Microsoft.Azure.Commands.Compute.Test.ScenarioTests.VirtualMachineTests.TestVMImageCmdletOutputFormat [1s 253ms]
2021-01-14T06:34:41.6940930Z EXEC : error Message:  [D:\a\1\s\build.proj]
2021-01-14T06:34:41.6941747Z      System.Management.Automation.RuntimeException : Assertion failed:  $output.Contains($str) 
2021-01-14T06:34:41.6942391Z   ---- System.Management.Automation.RuntimeException : Assertion failed:  $output.Contains($str) 
2021-01-14T06:34:41.6942810Z     Stack Trace:
2021-01-14T06:34:41.6943216Z        at System.Management.Automation.Runspaces.PipelineBase.Invoke(IEnumerable input)
2021-01-14T06:34:41.6943735Z      at System.Management.Automation.PowerShell.Worker.ConstructPipelineAndDoWork(Runspace rs, Boolean performSyncInvoke)
2021-01-14T06:34:41.6944361Z      at System.Management.Automation.PowerShell.Worker.CreateRunspaceIfNeededAndDoWork(Runspace rsToUse, Boolean isSync)
2021-01-14T06:34:41.6945300Z      at System.Management.Automation.PowerShell.CoreInvokeHelper[TInput,TOutput](PSDataCollection`1 input, PSDataCollection`1 output, PSInvocationSettings settings)
2021-01-14T06:34:41.6946108Z      at System.Management.Automation.PowerShell.CoreInvoke[TInput,TOutput](PSDataCollection`1 input, PSDataCollection`1 output, PSInvocationSettings settings)
2021-01-14T06:34:41.6946951Z      at System.Management.Automation.PowerShell.CoreInvoke[TOutput](IEnumerable input, PSDataCollection`1 output, PSInvocationSettings settings)
2021-01-14T06:34:41.6947554Z      at System.Management.Automation.PowerShell.Invoke(IEnumerable input, PSInvocationSettings settings)
2021-01-14T06:34:41.6948054Z      at System.Management.Automation.PowerShell.Invoke()
2021-01-14T06:34:41.6948775Z      at Microsoft.WindowsAzure.Commands.ScenarioTest.EnvironmentSetupHelper.RunPowerShellTest(String[] scripts) in D:\a\1\s\tools\ScenarioTest.ResourceManager\EnvironmentSetupHelper.cs:line 573
2021-01-14T06:34:41.6949654Z      at Microsoft.Azure.Commands.TestFx.TestManager.RunTestScript(String[] scripts) in D:\a\1\s\tools\TestFx\TestManager.cs:line 227
2021-01-14T06:34:41.7018545Z      at Microsoft.Azure.Commands.Compute.Test.ScenarioTests.VirtualMachineTests.TestVMImageCmdletOutputFormat() in D:\a\1\s\src\Compute\Compute.Test\ScenarioTests\VirtualMachineTests.cs:line 150