[Clang][NFC] Remove a redundancy check in Sema::adjustMemberFunctionCC

If the current calling convection CurCC is not equal to Target calling
convection ToCC and current calling convention (CurCC) is equal to the
default calling convention (DefaultCC), that means DefaultCC can not be
equal to ToCC so the right part of the expression DefaultCC == ToCC is
not needed.

revelant code -
  if (CurCC == ToCC)
    return;
  if (CurCC != DefaultCC || DefaultCC == ToCC)
    return;

Found by PVS-Studio - https://pvs-studio.com/en/blog/posts/cpp/1003/, N41.

Differential Revision: https://reviews.llvm.org/D142338