Drop stack Windows CI (#934)

The Stack Windows build is problematic:
  https://github.com/haskell/ghcide/pull/922

Stack is already covered by the Azure CI
Windows is already covered by the Github Actions CI

.azure/windows-stack.yml

@@ -1,41 +0,0 @@
-jobs:
-- job: ghcide_stack_windows
-  timeoutInMinutes: 120
-  pool:
-    vmImage: 'windows-2019'
-  variables:
-      STACK_ROOT: "C:\\sr"
-  steps:
-  - checkout: self
-  - task: Cache@2
-    inputs:
-      key: stack-root-cache | $(Agent.OS) | $(Build.SourcesDirectory)/stack-windows.yaml | $(Build.SourcesDirectory)/ghcide.cabal
-      path: $(STACK_ROOT)
-      cacheHitVar: STACK_ROOT_CACHE_RESTORED
-    displayName: "Cache stack root"
-  - task: Cache@2
-    inputs:
-      key: stack-work-cache | $(Agent.OS) | $(Build.SourcesDirectory)/stack-windows.yaml | $(Build.SourcesDirectory)/ghcide.cabal
-      path: .stack-work
-      cacheHitVar: STACK_WORK_CACHE_RESTORED
-    displayName: "Cache stack work"
-  - bash: |
-      ./fmt.sh
-    displayName: "HLint via ./fmt.sh"
-  - bash: |
-      curl -sSkL http://www.stackage.org/stack/windows-x86_64 -o /usr/bin/stack.zip
-      unzip -o /usr/bin/stack.zip -d /usr/bin/
-      mkdir -p "$STACK_ROOT"
-    displayName: 'Install Stack'
-  - bash: stack setup --stack-yaml stack-windows.yaml
-    displayName: 'stack setup'
-  - bash: |
-      # Installing happy and alex standalone to avoid error "strip.exe: unable to rename ../*.exe; reason: File exists"
-      stack install happy --stack-yaml stack-windows.yaml
-      stack install alex --stack-yaml stack-windows.yaml
-      choco install -y cabal --version=$CABAL_VERSION
-      $(cygpath $ProgramData)/chocolatey/bin/RefreshEnv.cmd
-      # GHC 8.10.1 fails with ghc segfaults, using -fexternal-interpreter seems to make it working
-      # There are other transient errors like timeouts downloading from stackage so we retry 3 times
-        stack build --test --no-run-tests --stack-yaml stack-windows.yaml --ghc-options="-fexternal-interpreter" || stack build --test --no-run-tests --stack-yaml stack-windows.yaml --ghc-options="-fexternal-interpreter" || stack build --test --no-run-tests --stack-yaml stack-windows.yaml  --ghc-options="-fexternal-interpreter"
-    displayName: 'stack build --test'

azure-pipelines.yml

@@ -15,4 +15,3 @@ pr:
 
 jobs:
   - template: ./.azure/linux-stack.yml
-  - template: ./.azure/windows-stack.yml