Bumped minimal CMake version to 2.8.7.
Removed CMake version checks that we don't need anymore.
Dropped OpenCVConfig.cmake - we don't use it anymore.
Removed OpenCVFindPkgConfig.cmake, since it's bundled with CMake now.

In fact, I don't think we actually use it... but I kept the include,
just in case.
Removed CMakeParseArguments.cmake, which we don't use.

It's part of CMake nowadays, too.
Removed obsolete CMake workaround.