unused codepaths in common/json regex parser

Static analysis says the following parser states are unreachable:

stateInRegexpPatternEsc in 'common/json/regexp.go'

stateInRegexpPatternEscU in 'common/json/regexp.go'

stateInRegexpPatternEscU1 in 'common/json/regexp.go'

stateInRegexpPatternEscU12 in 'common/json/regexp.go'

stateInRegexpPatternEscU123 in 'common/json/regexp.go'

This is probably just dead code, since it looks like this logic may be handled by regexp() later on in the file. I wanted to run this by people with more experience with the package to make 100% sure this isn't a real bug.