fix(mastodon): Use correct reference for secret name

This patch drops the usage of `common.names.fullname` as template, which
originates from the bitnami dependencies which fails if all dependencies
are disabled.

Since the relevant value that is intended to use is the name of the
secret from the helmchart itself, it makes no sense to use this subchart
as source, but rather use the same name as the secret name in this
chart.

References:
https://github.com/mastodon/chart/pull/60