Revert "update corelist with v5.18.1 changes"

This reverts commit f97801705daa638dc14b1ce38cac3542064df879.
import updates to Porting/corelist.pl
update corelist with v5.18.1 changes

...again.  This time, using the updated updater, fixing the
deprecations screwed up in 5.18.1 RC1.
perldelta: fix module updates section
perldelta: remove a leftover XXX section
RT #119125: continue /[#$x]/x not interpolating

the earlier fix for /[#](?{})/x, although correct, as a
side-effect fixed another long-standing bug where /[#$x]/x
didn't interpolate the var $x. Although fixing that is good,
it's too big a change for maint, so keep the old buggy behaviour
for now.
perlhist: add 5.18.1!
update perlhist and patchlevel for v5.18.1-RC2