Remove the HGitaly1 mode of native Mercurial
The HGitaly1 transitional mode is selected if the :hg_fully_native
feature flag is disabled.
The feature flag is enabled by default in Heptapod 0.28, the next logical move is to remove the HGitaly1 mode entirely.
Task list:
- remove testing of the HGitaly1 mode in heptapod/heptapod-tests>
- remove code checking the feature flag value
- remove the feature flag
- remove dead code