marge-bot: restrict marge-bot-mesa to mesa only

Currently the marge-bot-mesa release services not just the mesa project but also parallel-deqp-runner, gfx-ci/tracie, gfx-ci/traces-db, and gfx-ci/traces-db-private. But these other projects don't need to use the marge-bot-mesa release. The parallel-deqp-runner project is obsolete since it was superceded by deqp-runner. And all the projects in gfx-ci are serviced by the marge-bot-gfx-ci release.

So simplify the configuration of marge-bot-mesa by restricting it solely to the mesa project.