Remove base-mb platform

This base platform is no longer maintained.

For supporting the Microblaze CPU in the future, we might consider
integrating support for this architecture into base-hw. Currently
though, there does not seem to be any demand for it.

base/include/base/thread.h

@@ -230,9 +230,6 @@ namespace Genode {
 			 */
 			void _deinit_platform_thread();
 
-			/* hook only used for microblaze kernel */
-			void _init_context(Context* c);
-
 		protected:
 
 			/**