RATIS-2249. Remove unused shellcheck.sh (#1221) #4

Job	Run time
CI / build / build	1m 13s
CI / basic (checkstyle) / checkstyle	20s
CI / basic (findbugs) / findbugs	1m 46s
CI / basic (author) / author	12s
CI / basic (rat) / rat	16s
CI / unit (grpc) / unit (grpc)	11m 32s
CI / unit (misc) / unit (misc)	7m 7s
CI / unit (server) / unit (server)	12m 56s
CI / repro / repro	1m 9s
CI / compile (17) / compile (17)	1m 1s
CI / compile (11) / compile (11)	1m 3s
CI / compile (21) / compile (21)	1m 0s
CI / coverage	0s
	39m 35s

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