[libcxxabi] Remove the unused buildit script

Summary: I'm pretty sure it's not used anymore, at least it isn't used at Apple.

Reviewers: EricWF, Bigcheese

Subscribers: christof, jkorous, dexonsmith, jfb, mstorsjo, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D63297

llvm-svn: 363737