CLEANUP: regex: remove outdated support for regex actions

CLEANUP: regex: remove outdated support for regex actions

The support for reqrep and friends was removed in 2.1 but the
chain_regex() function and the "action" field in the regex struct
was still there. This patch removes them.

One point worth mentioning though. There is a check_replace_string()
function whose purpose was to validate the replacement strings passed
to reqrep. It should also be used for other replacement regex, but is
never called. Callers of exp_replace() should be checked and a call to
this function should be added to detect the error early.