Follow up to T368116

VisualEditor uses a LESS fade mixin to https://gerrit.wikimedia.org/g/VisualEditor/VisualEditor/+/7a92c5f8d441dde3cba79261850df61f0a60222c/src/ce/styles/nodes/ve.ce.BranchNode.less#69
both Vector 2022 and Minerva have skin styles to make this work in dark mode. Skin styles are usually a last resort and mean bugs can occur if we are not testing both themes and both skins when testing these, so I'd prefer we got these removed.

TODO

• Replace the fade mixin with a design token OR upstream the changes from Minerva and Vector
• Remove the relevant rules in skinStyles/ext.visualEditor.core.mobile.less,edit
• Remove the relevant rules in https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/bdb649dc7fbdd42e0fb18e8564abcf8b5997330d/skinStyles/ext.visualEditor.less#11

QA steps

• Visit https://en.m.wikipedia.beta.wmflabs.org/w/index.php?title=Polar_bear#/editor/all
• Hover at the start of the article
• Check new paragraph is readable