Add support for removing mods on entering gameplay

[felipemarins]

Addresses #17835.

Automatically removes mods on entering gameplay when they do nothing (e.g. difficulty adjust mod at default settings). For now, it only supports removing difficulty adjust mods. I'll try to implement the removal for key conversion mods later in another PR.

I'm just unsure about whether I should separate the test for each mod or just use this giant test.