[Non-normative] Rename intra_mode_to_tx_type_context
[Non-normative] Rename intra_mode_to_tx_type_context

The word context is confusing as it can be interpreted as EC context.
Removed it and added an assert to check that only intra modes are used.

Change-Id: Ibeb4ba3bed0fbf85cc20ec1d30a4862626acebcc

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