[ci] Remove obsolete QA repos. (#5908)

CI is failing with:

    The pipeline is not valid. Could not get the latest source version
    for repository xamarin/xamarin-forms-samples hosted on
    https://github.com/ using ref refs/heads/master. GitHub reported
    the error, "No commit found for SHA: refs/heads/master"

The `master` branch changed to `main` in xamarin/xamarin-forms-samples.

Several repos we are cloning in `azure-pipelines.yaml` are not actually
used anymore. We should just remove these to solve the issue.

Author: jpobst

build-tools/automation/azure-pipelines.yaml

@@ -29,21 +29,6 @@ resources:
     name: xamarin/UITools
     ref: refs/heads/main
     endpoint: xamarin
-  - repository: qa
-    type: github
-    name: xamarin/QualityAssurance
-    ref: refs/heads/master
-    endpoint: xamarin
-  - repository: samples
-    type: github
-    name: xamarin/monodroid-samples
-    ref: refs/heads/master
-    endpoint: xamarin
-  - repository: xfsamples
-    type: github
-    name: xamarin/xamarin-forms-samples
-    ref: refs/heads/master
-    endpoint: xamarin
 
 # Global variables
 variables: