3.x: don’t include editline/readline.h, use the embedded readline

[markmentovai]

When building for a Python whose own readline module has been configured to use editline, WITH_EDITLINE will be defined during the python-gnureadline build, by virtue of its inclusion of "pyconfig.h" (via "Python.h"). This macro being defined caused python-gnureadline to a problem because python-gnureadline is expecting and has configured itself to use its own embedded copy of GNU readline.

This removes the HAVE_EDITLINE branch to allow the expected readline headers to be included.

See https://trac.macports.org/ticket/68265.