configure: remove redundant alsa/asoundlib.h check

Originally committed as revision 17650 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2108,7 +2108,6 @@ check_header dev/ic/bt8xx.h
 check_header sys/soundcard.h
 check_header soundcard.h
 
-check_header alsa/asoundlib.h &&
 check_lib2 alsa/asoundlib.h snd_pcm_htimestamp -lasound
 
 # deal with the X11 frame grabber