[CMake] Remove dead FindPythonInterp code

LLVM has bumped the minimum required CMake version to 3.13.4, so this has become dead code.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D87189

GitOrigin-RevId: 86bd8f82cc74725a08a40efe176d3d6b9c9cef92
1 file changed