revert: remove unnecessary test infrastructure

Remove test_startup.sh, STARTUP_TEST.md, and var/.gitkeep as they are
not needed. The proper way to test qbot startup is simply using `nix run`.

The var directory is configurable and doesn't need to be preserved in git.
Only db/.gitkeep remains as the database needs a directory.

Co-authored-by: anna328p <9790772+anna328p@users.noreply.github.com>

Author: Copilot

.gitignore

@@ -13,7 +13,6 @@ config/global.yml
 **/yt-credentials.yml
 
 var/**
-!var/.gitkeep
 
 .idea