configure: remove special flags for XLC on Darwin

Rationale:
- Current versions of XLC accept gcc flags
- XLC can't compile FFmpeg

Originally committed as revision 19488 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1974,11 +1974,7 @@ check_deps $ARCH_EXT_LIST
 
 #Darwin CC versions
 if test $target_os = darwin; then
-    if enabled xlc; then
-        add_cflags -qpdf2 -qlanglvl=extc99 -qmaxmem=-1 -qarch=auto -qtune=auto
-    else
         enabled shared || check_cflags -mdynamic-no-pic
-    fi
 fi
 
 disabled optimizations || check_cflags -fomit-frame-pointer