Prefer erc--skip property to erc-track--skipped-msgs

Prefer erc--skip property to erc-track--skipped-msgs

* lisp/erc/erc-stamp.el (erc-add-timestamp): Honor an overriding
`erc--ts' "msg prop".
(erc-stamp--defer-date-insertion-on-post-modify): Add `erc--skip'
for the `track' module.
* lisp/erc/erc-track.el (erc-track--skipped-msgs): Remove unused
variable.  Originally added as part of bug#60936.
(erc-track-modified-channels): Rely on the `erc--skip' "msg prop"
instead of the now defunct `erc-track--skipped-msgs' variable for
detecting requests to omit `track' mode-line updates during
`erc-display-message'.