multimedia/mlt: remove SWFDEC from default OPTIONS

Swfdec has been abandonware for years and Flash Player has been losing marketshare anyway. Moreover, liboil (swfdec's dependency) doesn't build on powerpc64 elfv2, which blocks this port and in consequence kde5 metaport.

Approved by: tcberner (maintainer, mentor)
Differential Revision: https://reviews.freebsd.org/D22591