fix: resolve regression test for removeHiddenElems

[SethFalco]

We were skipping a regression test because one of our default plugins broke it.
This resolves the issue with the plugin, so the test doesn't have to be skipped anymore.

The problem was that it was detaching non-rendered elements from the document as well (mask), but non-rendered elements still apply even if they are set to be invisible. From manual testing, similar logic is needed for linearGradient too.

We already had a workaround for this that was specific to clipPath, this just expands it to work across non-rendered elements.

To preserve backward compatibility with existing tests, I added logic at the end that will still detach some non-rendered elements.

• If it doesn't have an ID, so can't be referenced elsewhere in the document.
• If after cleaning up, no nodes are referencing the ID of the non-rendered element.

Metrics

Executed with only preset-default enabled:

SVG	Time	% reduced	final size
Original	N/A	N/A	15060.532 KiB
v3.0.2	14915 ms	31.4%	10326.444 KiB
main (bd750ce)	14641 ms	31.4%	10326.567 KiB
this branch	18357 ms	31.4%	10326.567 KiB