Removed references to nonexistant mp3 files

This revision does not contain the mp3 reading files. I thought I had
ammended my last commit to not include those files, but I guess I
didn't.

Change-Id: Ied893c32d717fdc256c945a3086ce8b4ca5fc1da

CMakeLists.txt

@@ -22,15 +22,13 @@ set(SDL_MIXER_SRCS
     dynamic_fluidsynth.c
     dynamic_mod.c
     dynamic_modplug.c
-    dynamic_mp3.c
     dynamic_ogg.c
     effect_position.c
     effects_internal.c
     effect_stereoreverse.c
     fluidsynth.c
     load_aiff.c
     load_flac.c
-    load_mp3.c
     load_ogg.c
     load_voc.c
     mixer.c