python-utils-r1.eclass: python_wrapper_setup, do not query PYTHON_CONFIG

python-utils-r1.eclass: python_wrapper_setup, do not query PYTHON_CONFIG

Use plain 'pythonX.Y-config' executable name in the python-config
wrapper instead of querying the complete PYTHON_CONFIG path from
python_export.

Constructing full PYTHON_CONFIG path requires querying ABIFLAGS which
requires the Python interpreter to be installed, which in turn
unnecessarily forces every ebuild using the eclass to carry a build-time
dependency on the Python interpreter.

All current versions of Python correctly install 'pythonX.Y-config'
symlink that points to the correct ABI-suffixed binary. It is therefore
unnecessary to use the full path.