Remove `dbg.regression` option #14625

Job	Run time
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	24m 48s
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	7m 44s
extraction	0s
	32m 32s

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