[release/2.1] Clean up YAML slightly (#29737)

- get `buildArgs` parameter to default-build.yml template working
    - nothing uses this at the moment (now that pr-validation-temp.yml is no more)
    - but, it's documented and can be useful
- remove unused `buildScript` parameter
- remove unused project-ci.yml template

Author: dougbu

.azure/pipelines/jobs/default-build.yml

@@ -54,6 +54,7 @@ parameters:
     publish: true
     path: 'artifacts/'
   buildDirectory: ''
+  buildArgs: ''
 
 jobs:
 - job: ${{ coalesce(parameters.jobName, parameters.agentOs) }}
@@ -84,7 +85,7 @@ jobs:
         queue: BuildPool.Server.Amd64.VS2017
   variables:
     AgentOsName: ${{ parameters.agentOs }}
-    BuildScript: ${{ parameters.buildScript }}
+    BuildScriptArgs: ${{ parameters.buildArgs }}
     BuildConfiguration: ${{ parameters.configuration }}
     BuildDirectory: ${{ parameters.buildDirectory }}
     BinlogArg: /bl:artifacts/logs/${{ parameters.agentOs }}.binlog
@@ -186,4 +187,4 @@ jobs:
   - ${{ if and(eq(variables['System.TeamProject'], 'internal'), eq(parameters.agentOs, 'Windows')) }}:
     - task: MicroBuildCleanup@1
       displayName: Cleanup MicroBuild tasks
-      condition: always()
+      condition: always()