[clang][parser] Don't prohibit attributes on objc @try/@throw

This line has a TODO comment, but the answer to it seems to be "no"
given that clang itself uses attributes on @try statements in its tests.

This ProhibitAttributes() statement is also dead code since
ProhibitAttributs() does not handle GNU attributes at the moment but
those are the only attributes valid in objc.

Differential Revision: https://reviews.llvm.org/D97371