[Remarks] Remove yaml-strtab format (#144527)
Background: The yaml-strtab format looks just like the yaml format,
except that the values in the key/value pairs of the remarks are
deduplicated and replaced by indices into a string table (see removed
test cases for examples). The motivation behind this format was to
reduce size of the remarks files. However, it was quickly superseded by
the bitstream format.
Therefore, remove the yaml-strtab format, as it doesn't have a good
usecase anymore:
- It isn't particularly efficient
- It isn't human-readable
- It isn't straightforward to parse in external tools that can't use the
remarks library. We don't even support it in opt-viewer.
llvm-remarkutil is also missing options to parse/convert yaml-strtab, so
the chance that anyone is actually using this format is low.
