Remove unused headers in regexp_dfa.cc. #10246

Job	Run time
Run e2e tests (uninstrumented)	3m 31s
Run unit tests (opt)	2m 11s
Run e2e tests (libfuzzer)	3m 56s
Run unit tests (dbg)	2m 1s
Run e2e tests (fuzztest-experimental)	11m 4s
Run e2e tests (fuzztest)	5m 10s
Run unit tests (fastbuild)	1m 45s
	29m 38s

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