To fix the runtime crashes in difficulty calculation components, remove
the check for pre-existing fallback registration along with
the exception. The xmldoc for the registration function has been
extended to make users aware of possible consequences of calling it.
LegacyDifficultyCalculatorBeatmapDecoder was registered as a fallback
decoder in commit ffde389 for future use in the server-side difficulty
calculation components. Due to the pre-existing fallback registrations
this causes a runtime crash when the diffcalc components are started.
Add a test reproducing this scenario to prevent the issue from
resurfacing in the future.