Remove an invalid assertion in av1_encode_block_intra()
Remove an invalid assertion in av1_encode_block_intra()

Also add a TODO in encode_block() for txk type check.

BUG=aomedia:1551

Change-Id: Ia020335b40dffc7eb5d313cb213588ba2781cea4

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom