Revert "Add new attribute 'objc_suppress_protocol' to suppress protocol conformance for a class."

After implementing this patch, a few concerns about the language
feature itself emerged in my head that I had previously not considered.
I want to resolve those design concerns first before having
a half-designed language feature in the tree.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195328 91177308-0d34-0410-b5e6-96231b3b80d8