Remove LinuxImageDemand variable and replace usage with actual string value (Azure#41734)

Author: Copilot

eng/pipelines/docindex.yml

@@ -28,7 +28,7 @@ jobs:
   - job: UpdateDocsMsBuildConfig
     pool:
       name: $(LINUXPOOL)
-      demands: $(LinuxImageDemand)
+      demands: ImageOverride -equals $(LINUXVMIMAGE)
     variables:
       DocRepoLocation: $(Pipeline.Workspace)/docs
       DocRepoOwner: MicrosoftDocs

eng/pipelines/templates/variables/image.yml

@@ -25,5 +25,4 @@ variables:
   - name: MACOS
     value: macOS
 
-  - name: LinuxImageDemand
-    value: ImageOverride -equals $(LINUXVMIMAGE)
+