ios: use ffmpeg instead of libav

FFmpeg provides more hardware acceleration than libav for iOS

Since FFmpeg is now the default provider for libav*, remove all
the ifdef clauses. For platforms using libav, an ifdef should be
added to the rules.mak

Change-Id: I6f53bdf35b16803aa0a3f5aeda9c7c430cd0025d

contrib/src/ffmpeg/rules.mak

 FFMPEG_HASH := n3.3.1
 FFMPEG_URL := https://git.ffmpeg.org/gitweb/ffmpeg.git/snapshot/$(FFMPEG_HASH).tar.gz
 
-ifdef HAVE_WIN32
-PKGS += ffmpeg
-endif
-
-ifdef HAVE_MACOSX
 PKGS+=ffmpeg
-endif
-
-ifdef HAVE_LINUX
-PKGS += ffmpeg
-endif
 
 ifeq ($(call need_pkg,"libavcodec >= 57.89.100 libavformat >= 57.71.100 libswscale >= 4.6.100 libavdevice >= 57.6.100 libavutil >= 55.58.100"),)
 PKGS_FOUND += ffmpeg

contrib/src/libav/rules.mak

 LIBAV_HASH := v12.1
 LIBAV_URL := https://git.libav.org/?p=libav.git;a=snapshot;h=$(LIBAV_HASH);sf=tgz
 
-ifndef HAVE_LINUX
-ifndef HAVE_WIN32
-ifndef HAVE_MACOSX
-PKGS += libav
-endif
-endif
-endif
-
 #disable everything
 #ensure to add this option first
 LIBAVCONF = \