[CMake] Fix Python 3 lookup when building LLVM with tests

Remove the find_package(Python3 ...) call from Tooling/CMakeLists.txt as
it would override the python 3 version determined in llvm/CMakeLists.txt.
This call did not respect the LLVM_MINIMUM_PYTHON_VERSION.

This fixes the check-all target when building LLVM on a system where the
default python version is not the minimum required version for running tests.

Reviewed By: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D99715