[libc++] Remove old workaround for buildit

Summary:
I'm not sure what the problem was at the time, however I don't think
this is necessary since buildit doesn't exist anymore.

Instead of the workaround, the correct thing to do is to leave out
the get_new_handler/set_new_handler definitions from libc++ when
we're getting them from libc++abi.

Reviewers: EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60717

llvm-svn: 358518