media-libs/opencv: Kill unnecessary Gentoo hack-patch for Python

media-libs/opencv: Kill unnecessary Gentoo hack-patch for Python

Kill the unnecessary, outdated custom Gentoo CMake module used to force
specific version of Python in favor of hacking the upstream module
appropriately. Rely on the standard CMake -DPYTHON_INTERPRETER to
(indirectly) disable Python for the base build, and on python-r1 +
upstream -DPYTHON3_INTERPRETER to control the implementation used for
Python module build.

This removes a lot of unnecessary hackery from the ebuild, such as
passing all the Python implementation specifics (include paths, library
names) in favor of using the CMake autodetection code. It also restores
some missing bits that were omitted because of the Gentoo module
deficiency.

To keep things simple, all versions of Python are treated as 'python2'
upstream since the code used to build both python2 & python3 modules is
identical (i.e. works for all versions).