Removed last TreePlus.gif and TreeMinus.gif spots

[agh1]

Overview

This kills off the last few uses of the little black triangles and deletes them (and the little white triangles that weren't actually used for a while now). It's a grab bag of uses here, but most are this pattern in the screenshot where there's a link to add a new thing and a link to remove one. They're more properly reflected with the plus sign and trash can anyway

This will need to be merged after #17295 and #17296 since they remove uses of files that this PR removes.

Before

After

[civibot]

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[agh1]

Jenkins retest this please

[agh1]

I updated this with aria-hidden per #17318

[eileenmcnaughton]

test this please