[cmake] Remove redundant find_package(Python)

Summary: These calls are no-op because the FindPython for the interpreter is already called in the main CMakeLists.txt (and the variable is cached). Also these calls don't make it mandatory nor enforce a minimum version which is confusing.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16947