[build] stop setting $DOTNET_gcServer (#8085)

Context: https://github.com/dotnet/linker/issues/3012
Context: dotnet/runtime#86807

In 34986b9, we exported `DOTNET_gcServer=0` so that the Server GC is disabled, in an
effort to prevent an `AccessViolationException`.

It appears that this might be *fixed* in .NET 8. Let's remove the workaround, so we
can either close the issue or get further investigation on the issue.

build-tools/automation/yaml-templates/variables.yaml

@@ -40,9 +40,6 @@ variables:
 # Workaround: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1585820
 - name: _WriteTelemetryProperties
   value: false
-# Workaround: https://github.com/dotnet/linker/issues/3012
-- name: DOTNET_gcServer
-  value: 0
 - name: IsRelOrTargetingRel
   value: $[or(startsWith(variables['Build.SourceBranch'], 'refs/heads/release/'), startsWith(variables['System.PullRequest.TargetBranch'], 'release/'))]
 - name: DotNetNUnitCategories