Move useInterpolatedNLSFs assert from encode_indices.c to silk/process_NLSFs.c.

There were assert failures for test cases with mode switching and LBRR enabled.
(some similar problems remain)

Tim identified that the assert was falsely firing while encoding LBRR frames but
the flag doesn't matter while encoding the bitstream it only actually matters in
silk/process_NLSFs.c.