Remove outdated assert
Remove outdated assert

As of 3491dd292177d34575236fb4bd387f866a5736e0,
av1_twopass_postencode_update can be called for all frame types.

BUG=aomedia:2103

Change-Id: I36120a5996629cfbb3e1c3c49b27a8bbbd58977c

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