[regex] Remove dead code

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

This patch removes several unused parameters and struct fields.

The most egregious is the "flag" struct re_backref_cache_entry field.
Removing it turned a potentially quadratic loop into linear, though I did
not try to find a case in which this actually happened.

In another case, it looks like changing the algorithms would have required
changing the node_set passed to sift_states_bkref, but this was not done.
So this patch does that instead of only removing the unused parameter.

Paolo

Attachment: 02-regex-cleanup-un#5CB76.patch
Description: Binary data