tests/ci: rely on pre-existing *_CONF make variables

[guest-svmhdvn]

Summary

This patch removes the unnecessary MAKECONF and SRCCONF
from tests/ci/Makefile and relies on the following
pre-existing standard variables to control its configuration:

__MAKE_CONF for make.conf
SRCCONF for src.conf
SRC_ENV_CONF for src-env.conf

This also removes the WITH_META_MODE custom logic, since that
can easily be controlled through SRC_ENV_CONF already.

Signed-off-by: Siva Mahadevan <[e-mail]>
Sponsored by: The FreeBSD Foundation

[bofh]

I don't think that this is correct at this moment. The reasons are:

1. This patch(MAKECONF/SRCCONF) will be helpful when we fully control the different ci tasks from the root Makefile which is up for future discussion
2. The primary reason we add these logics(METAMODE) are so that a developer has to do less when they are running their pre-commit CI checklist at their terminal or choice of environment

The primary task for us is to make running ci easier for end developers not our CI tasks. But I believe @imp can shade some more light onto the concept.

I understand that you are trying to make some improvements here but before that you should actually read/understand/realize why we started this project or what was the rationale behind this. Ask @jrm for a copy of the original proposal from 2022. Time has changed and there are new variables in the play but the goal should be same.

[guest-svmhdvn]

Abandoning due to feedback.