xbase/shl.mi: Delete, rather than obsolete, libXxf86misc.so.M(.N).

Obsolete lines here will lead postinstall to delete the shlib, which
will break any existing applications linked against it.  Deleting the
lines has no effect on postinstall.

(We should maybe have a different way to mark them instead, as
suggested in PR 57581, but we don't, so this is what we do for now.)

However, keep the obsolete line for libXxf86misc.so, since the .so
symlink itself is only used when linking new applications.

(riastradh)