Dolphin already uses QStyleHints::colorScheme to detect dark mode,
so this code is now redundant.

Also clean up signals a bit.

This lets us reduce the number of USE_RETRO_ACHIEVEMENTS ifdefs in the
code base, reducing visual clutter. In particular, needing an ifdef for
each call to IsHardcodeModeActive was annoying to me. This also reduces
the risk that someone writes code that accidentally fails to compile
with USE_RETRO_ACHIEVEMENTS disabled.

We could cut down on ifdefs even further by making HardcodeWarningWidget
always exist, but that would result in non-trivial code ending up in the
binary even with USE_RETRO_ACHIEVEMENTS disabled, so I'm leaving it out
of this PR. It's not a lot of code though, so I might end up revisiting
it at some point.

Bugfix for hardcore-disabled items being disabled when hardcore was true but achievement integration was false, which should mean hardcore is effectively disabled. Now everything checks the IsHardcoreModeActive method in AchievementManager which processes the setting AND the game state to determine if hardcore mode is actually active.