Fix regression in `get_tag_link()` since 4.9.

See [42364] for description of the problem.

Because `get_category_link()` is now totally taxonomy-agnostic, `get_tag_link()`
can become a simple wrapper.

Props juiiee8487, markjaquith.
See #42771.
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git-svn-id: https://core.svn.wordpress.org/trunk@42195 1a063a9b-81f0-0310-95a4-ce76da25c4cd