Remove overlay/ghc.nix (#2340) #3012

Job	Run time
cabal-doExactConfig	54s
runghc	52s
nix-build	2m 39s
maintainer-scripts	7s
plan-extra-hackages	55s
multi-target	1m 23s
shellFor-hoogle	55s
shellFor-multiple-package	44s
shellFor-not-depends	17s
shellFor-single-package	19s
tests-benchmarks	10s
build-extra-hackages	6s
cabal	9s
check-materialization-concurrency	20s
check-path-support	10s
closure-size	2s
docs	37m 57s
haskell-nix-roots-do-not-require-IFDs	1m 9s
hix	26s
hydra-ifdLevel-0-and-1	4m 27s
hydra-ifdLevel-2	11m 55s
hydra-ifdLevel-3	30m 8s
hydra-without-remote-builders-ghc966	1m 3s
unit-tests	4s
update-docs	5s
hix-cabal	12s
hydra-without-remote-builders-ghc9101	3m 47s
nix-tools	2m 52s
	1h 44m 7s

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