Now MySQL is no longer supported, we need to change the docs

Signed-off-by: Bastian Blank <waldi@debian.org>

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This is useful for when runner is bundled with gitlab, like in a kubernetes stack, and we want the runner to be able to
register with gitlab as soon as they both come up.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

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This makes it easier for users to use their own limits based on their
server configuration.