All CI used to go through scripts/generic-ci-tests.sh, which is a
wrapper around various `paver` test/linting/check invocations.
These days, most edx-platform CI checks just invoke their tools (pylint,
pycodestyle, pytest, etc.) directly.
In anticipation of the proposed Paver deprecation [1], let's remove
the parts of this script that aren't used any more, including several
`paver` command invocations. This should have no impact on CI.
Furthermore, we are able to remove the SHARD environment variable,
which was formely used to split unit and quality checks up into
smaller pieces. Unit tests and pylint checks now have their own
separate sharding logic, so there is only one "quality" shard remaining
(SHARD=4, ie generic quality checks), thus we don't need a SHARD
variable at all.
[1] https://github.com/openedx/edx-platform/issues/34467
5a785482ab