searcher: don't copy regular expressions

Created by: nicksnyder

This is no longer necessary as of go 1.12 according to this staticcheck message:

SA1019: rg.re.Copy is deprecated: In earlier releases, when using a Regexp in multiple goroutines, giving each goroutine its own copy helped to avoid lock contention. As of Go 1.12, using Copy is no longer necessary to avoid lock contention. Copy may still be appropriate if the reason for its use is to make two copies with different Longest settings. (staticcheck)