Don't force using the system libunwind for source-build (#58911)

Instead, use the runtime's normal detection logic to decide what to do.

Backport of #58641 to release/6.0.

See: dotnet/source-build#2408

Author: omajid

eng/SourceBuild.props

@@ -40,7 +40,6 @@
       <InnerBuildArgs>$(InnerBuildArgs) --verbosity $(LogVerbosity)</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) --nodereuse false</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) --warnAsError false</InnerBuildArgs>
-      <InnerBuildArgs>$(InnerBuildArgs) --cmakeargs -DCLR_CMAKE_USE_SYSTEM_LIBUNWIND=TRUE</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) /p:MicrosoftNetFrameworkReferenceAssembliesVersion=1.0.0</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) /p:PackageRid=$(TargetRid)</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) /p:NoPgoOptimize=true</InnerBuildArgs>