Remove support for scalar quantization

Reasons are:
- No quantization matrix support (and none planned) so impossible to have a
  reasonable comparison with PVQ anyway
- We need to reduce the number of interactions between OD_DISABLE* flags
- We need to tune tools for PVQ rather than scalar, which will make scalar worse
- The new intra and CfL patches can't work with scalar

Conflicts:
	src/decode.c
	src/encode.c
Simplifies CfL by signalling the gain and sign through PVQ