Remove `CompositeMCMCFitter` #4853

Job	Run time
macos-latest-py313	1h 7m 54s
ubuntu-latest, ephemeris_connection	1m 38s
ubuntu-latest, black	17s
ubuntu-latest, py313-test-cov	1h 28m 45s
ubuntu-latest, notebooks	5m 12s
ubuntu-latest, oldestdeps	29m 22s
	3h 13m 8s

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