BSC_Tests: use c_mr_conf_5_90 in TC_assignment_codec_amr_startmode_cruft

This way it's easier to see that the only difference from other
tests using c_mr_conf_5_90 is the ICMI field set to false.

Change-Id: If7b491fa55a9366520c2d665a700c5badb187fae
1 file changed