Remove old ffpresets.

They are now replaced with presets/ directory. WIN32 still seems to use
a ffpresets/ directory, but it doesn't look like to be deployed at
install time.

ffpresets/libvpx-1080p.ffpreset

-vcodec=libvpx
-g=120
-rc_lookahead=16
-quality=good
-speed=0
-profile=1
-qmax=51
-qmin=11
-slices=4
-vb=2M
-
-#ignored unless using -pass 2
-maxrate=24M
-minrate=100k
-arnr_max_frames=7
-arnr_strength=5
-arnr_type=3

ffpresets/libvpx-1080p50_60.ffpreset

-vcodec=libvpx
-g=120
-rc_lookahead=25
-quality=good
-speed=0
-profile=1
-qmax=51
-qmin=11
-slices=4
-vb=2M
-
-#ignored unless using -pass 2
-maxrate=24M
-minrate=100k
-arnr_max_frames=7
-arnr_strength=5
-arnr_type=3

ffpresets/libvpx-360p.ffpreset

-vcodec=libvpx
-g=120
-rc_lookahead=16
-quality=good
-speed=0
-profile=0
-qmax=63
-qmin=0
-vb=768k
-
-#ignored unless using -pass 2
-maxrate=1.5M
-minrate=40k
-arnr_max_frames=7
-arnr_strength=5
-arnr_type=3

ffpresets/libvpx-720p.ffpreset

-vcodec=libvpx
-g=120
-rc_lookahead=16
-quality=good
-speed=0
-profile=0
-qmax=51
-qmin=11
-slices=4
-vb=2M
-
-#ignored unless using -pass 2
-maxrate=24M
-minrate=100k
-arnr_max_frames=7
-arnr_strength=5
-arnr_type=3

ffpresets/libvpx-720p50_60.ffpreset

-vcodec=libvpx
-g=120
-rc_lookahead=25
-quality=good
-speed=0
-profile=0
-qmax=51
-qmin=11
-slices=4
-vb=2M
-
-#ignored unless using -pass 2
-maxrate=24M
-minrate=100k
-arnr_max_frames=7
-arnr_strength=5
-arnr_type=3