Remove LinuxImageDemand and WindowsImageDemand variables from pipeline configuration (#45808)

Author: Copilot

eng/pipelines/docindex.yml

@@ -28,7 +28,7 @@ jobs:
   - job: UpdateDocsMsBuildConfig
     pool:
       name: $(LINUXPOOL)
-      demands: $(LinuxImageDemand)
+      demands: ImageOverride -equals $(LINUXVMIMAGE)
 
     timeoutInMinutes: 180
     variables:

eng/pipelines/templates/variables/image.yml

@@ -16,6 +16,3 @@ variables:
     value: windows-2022
   - name: MACVMIMAGE
     value: macos-latest
-
-  - name: LinuxImageDemand
-    value: ImageOverride -equals $(LINUXVMIMAGE)