(rcirc-markup-attributes): Delete markup codes straight after
processing it, to correctly handle toggling codes. E.g. when
processing the message "one ^Btwo^B three", we previously
recognised the first interval "^Btwo^B" and moved over the ^B.
Searching for the next range to markup, we would now falsely
recognise "^B three" to be another markup range, instead of it
being the negative toggle. By removing the processed codes, we
avoid this issue. This reverts the functionality removed in
946ceca2, and therefore also re-adds the check to make an
exception to removing a ^O as that code can terminate multiple
markup sequences. (Bug#57383)
2777178dc6