Palette code: remove the use of same if condition twice.
Palette code: remove the use of same if condition twice.

rd_pick_palette_intra_sby() method is called only when,
cpi->common.allow_screen_content_tools is on. So, no need to check that
again. We just use an assert() instead to still be safe.

Change-Id: I19785c2aac016798c8d331bbe91971b3806b73a8

mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,smarter/aom