Fix ILLink running in incremental builds (#77110)

Now that the new APICompat runs after all the compilation targets, this depends on in illink.targets isn't necessary anymore and fixes the incremental build issue.

Regressed with 960e4d7 as the new APICompat doesn't run as post compiler target anymore but instead, as part of the `PrepareForRun` target. That caused the ILLink dependent target to always run, even if the compiler didn't run and the inputs/outputs didn't change.

eng/illink.targets

@@ -3,8 +3,6 @@
     <IsTrimmable Condition="'$(IsTrimmable)' == ''">true</IsTrimmable>
     <PrepareResourcesDependsOn>_EmbedILLinkXmls;$(PrepareResourcesDependsOn)</PrepareResourcesDependsOn>
     <TargetsTriggeredByCompilation Condition="'$(DesignTimeBuild)' != 'true'">$(TargetsTriggeredByCompilation);ILLinkTrimAssembly</TargetsTriggeredByCompilation>
-    <!-- ApiCompat's assembly validation should use the trimmed assemblies as an input, so we make sure that ILLink runs first. -->
-    <ApiCompatValidateAssembliesDependsOn Condition="'$(IsCrossTargetingBuild)' != 'true'">$(ApiCompatValidateAssembliesDependsOn);ILLinkTrimAssembly</ApiCompatValidateAssembliesDependsOn>
   </PropertyGroup>
 
   <!-- Flow the IsTrimmable property down to consuming projects, in order for oob.proj