Remove Action::ObjcActOnMethodDefinition(). Rationale:

- It is not an "action" - it is never called by the parser.
- It was only used by one method, Sema::ObjcActOnStartOfMethodDef().

As a result, the logic it embodied is now directly implemented in Sema::ObjcActOnStartOfMethodDef().

llvm-svn: 44008