Revert "[Core] Add logging in case we find a non-existent target framework"

This reverts commit 51c4cc8.

Author: Therzok

main/src/core/MonoDevelop.Core/MonoDevelop.Core.Assemblies/TargetRuntime.cs

@@ -247,7 +247,6 @@ protected TargetFrameworkBackend GetBackend (TargetFramework fx)
 					return backend;
 				backend = fx.CreateBackendForRuntime (this);
 				if (backend == null) {
-					LoggingService.LogError ("TargetFramework creation fallback for framework: {0}", fx.Name);
 					backend = CreateBackend (fx);
 					if (backend == null)
 						backend = new NotSupportedFrameworkBackend ();