[Port dspace-8_x] Remove unused PostCSS plugins

[tdonohue]

Description

Manual port of #3547 to dspace-8_x. See details in that PR.

Tested this port PR manually & ensured there are no bugs introduced in 8.x themes. No issues found.

[kshepherd]

@tdonohue there are a few version bumps in yarn.lock as well, was that intentional?

[tdonohue]

@kshepherd : All the changes to yarn.lock were automated by running yarn remove [dependency] (e.g. yarn remove postcss-apply). So, the version changes you see were made by yarn as part of removing these dependencies. If you look closely, the yarn.lock changes are just removing some old versions of other dependencies which were used by these PostCSS plugins. (This is a good thing because it shows removing these PostCSS plugins removes other older dependencies from our yarn.lock)

So, the changes to yarn.lock were made by yarn itself. In that manner, they were intentional, because yarn saw necessary cleanup to make. :) Hopefully that makes more sense!

[kshepherd]

approving and merging backport of already-merged commit, tests well