mpd-full: remove partial BUILD_PATENTED on ffmpeg

mpd-full: remove partial BUILD_PATENTED on ffmpeg

This removes the guard "CONFIG_BUILD_PATENTED" for the ffmpeg build
feature for mpd-full. ffmpeg was already unconditionally registered as a
dependency (at line 51).

The ffmpeg package already handles whether or not to include patented
codecs internally using the same config flag, so guarding its use as a
dependency here does not appear necessary anymore.

(For reference: guard "BUILD_PATENTED" was introduced in 44eb12f for
ffmpeg, then partially removed by 326c973).

Signed-off-by: Pacien TRAN-GIRARD <pacien.trangirard@pacien.net>