Ignore embedding database in truncate Rake task

What does this MR do and why?

Previously running the following:

bin/rake gitlab:db:lock_writes
bin/rake gitlab:db:truncate_legacy_tables:ci

would result in an error due to the tanuki_bot_mvc table not being locked for writes. This table is the embedding database and can be ignored in this Rake task.

Relates to gitlab-development-kit#1640 (closed)

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