Fix possible bug with SysTimer

[c1728p9]

Ensure the SysTimer isn't added to the timer list twice by adding an extra call to remove() inside suspend().

Description

@stevew817 has reported a problem with SysTimer which this PR fixes.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[c1728p9]

Note - I have not actually been able to reproduce this failure, so it may not affect the latest master.

[c1728p9]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2170
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7027/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1958
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7027/1958

[mbed-ci]

Exporter Build : FAILURE

Build number : 1794
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7027/

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1812
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7027/