[CMake] Remove duplicated logic to find Python when doing a standalone build

I'm pretty sure there's no need to have this logic living in
LLDBStandalone. It doesn't appear anything in LLVM depends on this, and
We always go through LLDBConfig.cmake which has the canonical way to
find the Python libs and interpreter for LLDB.

Differential revision: https://reviews.llvm.org/D64821

llvm-svn: 366363