lang/python3[89]: remove LIBFFI option

Python now requires libffi from ports and does not build with LIBFFI
disabled, so remove the option.

PR: 256141
Reported by: majo-bugs.freebsd.org@cerny.sk
Reviewed by: koobs (python)
Approved by: koobs (python)
MFH: 2020Q2 (bugfix)