Fix assertion in lv_map experiment

Turn off the assertion because av1_init_lv_map doesn't pass the
count according to the position into get_base_ctx_from_count_mag()

The speeding-up optimize_txb is still on-going.
I will turn the assertion on once the speeding-up process is
getting to a milestone.

BUG=aomedia:704

Change-Id: I0119bf825bedc1fccbe4963f341a17f0e2601d02