Tags meta box: consider removing taghint

[afercia]

Noticed today on Slack with @michaelarestad and @azaozz, looks like taghint was used in 3.0 (and maybe later, see screenshot) like a "fake placeholder" for the new tag input field but it doesn't work anymore (HTML is changed, always hidden with CSS) and maybe no more needed. Still takes some bytes of JavaScript and CSS though. Worth considering some cleaning?
See also Press This.

[helen]

I wondered to myself if using an HTML placeholder would make sense here, but I think with that multi-purpose input (single and multiple entry) and extra helper text below, it's better without the visual noise. Nobody seems to have complained previously, anyway :)

[welcher]

Removes the taghint div and adjust css margin accordingly

[welcher]

Updated patch to remove additional css/js references

[welcher]

Removed unintended changes in patch

[helen]

In 31607:

Remove the once-placeholder-esque "tag hint", which has not worked in quite some time.

It's not particularly useful, and adds visual noise when in place.

props welcher.
fixes #31485.