mail/neomutt: prune the remains of SCREEN

Cleanup leftovers after S-lang support was removed upstream
and was excluded from the port at revision

https://cgit.freebsd.org/ports/commit/?id=05bfdaa815652f3f588e63ab0d4c036c0eeadc68

Signed-off-by: Eygene Ryabinkin <rea@FreeBSD.org>