4.x major: Remove deprecations, fix up MultiChecker. #801

Job	Run time
cs-stan / Coding Standard & Static Analysis	33s
testsuite / testsuite-linux (8.2, highest, false)	37s
testsuite / testsuite-linux (8.5, highest, false)	1m 20s
testsuite / testsuite-linux (8.4, highest, false)	1m 18s
testsuite / testsuite-linux (8.2, lowest, false)	32s
testsuite / testsuite-linux (8.3, highest, false)	32s
	4m 52s

Provide feedback