Remove the home-grown android toolchain file and all references to it

Summary:
The toolchain file has been deprecated in favor of the "official"
toolchain file present in the Android NDK. Also update the web
build instructions to reflect this.

Reviewers: eugene

Subscribers: srhines, mgorny, dgross, tberghammer, lldb-commits

Differential Revision: https://reviews.llvm.org/D32441

llvm-svn: 301306