ac3parse: remove unused variable

ac3parse: remove unused variable

remove unused variable to fix compile error:
make -C audioparsers
make[3]: Betrete Verzeichnis '/home/lex/tmp/gst-plugins-good/gst/audioparsers'
  CC     libgstaudioparsers_la-gstaacparse.lo
gstaacparse.c: In function 'gst_aac_parse_read_loas_audio_specific_config':
gstaacparse.c:446:12: error: variable 'sbr' set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors

Signed-off-by: Oleksij Rempel (Alexey Fisher) <bug-track@fisher-privat.net>