regex: Remove in-tree version

The regex library was largely used for non-critical aspects of the compiler and
various external tooling. The library at this point is duplicated with its
out-of-tree counterpart and as such imposes a bit of a maintenance overhead as
well as compile time hit for the compiler itself.

The last major user of the regex library is the libtest library, using regexes
for filters when running tests. This removal means that the filtering has gone
back to substring matching rather than using regexes.

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