Drop a proposed commit which isn't applicable to maint

As noted by Daniel Dragan, the #123339 fix came up during work on the fix
for the very old #40565, but was a fix for a more recent blead problem
which maint doesn't have.
Drop more proposed commits

As Father C has noted, this sequence of commits is not a necessary part of
the fix for #122747. Therefore, they should be dropped since they don't
otherwise meet the perlpolicy critera for backporting. (And as Daniel
Dragan noted, the mathomisation change would also break binary
compatibility.)
Drop a proposed commit which is already in maint

This slipped in between 5.20.1-RC1 and 5.20.1-RC2, which I somehow missed
when compiling the list for 5.20.2. Thanks to Father C for the spot.