Disable multicore jitting

We've seen crashes that appear to be involved with multicore JIT that
only started soon after multicore JIT was enabled for the compiler
(tracked in #23846). This change disables multicore JIT until we can fix
the root cause of the crashes.

src/Compilers/Shared/BuildClient.cs

@@ -86,11 +86,6 @@ internal RunCompilationResult RunCompilation(IEnumerable<string> originalArgumen
                 return RunCompilationResult.Failed;
             }
 
-            if (!TryEnableMulticoreJitting(out errorMessageOpt))
-            {
-                textWriter.WriteLine(errorMessageOpt);
-            }
-
             if (hasShared)
             {
                 sessionKeyOpt = sessionKeyOpt ?? GetSessionKey(buildPaths);