`magit-status-insert-tags-line': remove redundant option

`magit-insert-status-tags-line' can now be enabled/disabled in
`magit-status-insert-sections-hook' (see commit 9da68462), so the
`magit-status-insert-tags-line' option no longer serves any purpose.

As a side effect, it's now enabled by default, which is a good thing IMO.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #904 from praet/insert-tags-line

`magit-status-insert-tags-line': remove redundant option