[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@363737 91177308-0d34-0410-b5e6-96231b3b80d8