Revert "Account for leading timestamps in erc-match"

Revert "Account for leading timestamps in erc-match"

This reverts commit 99d74dcd45938e2686d93eb5649800e14a88cd84 but keeps
the test file test/lisp/erc/erc-scenarios-match.el.  It also
implements a partial alternative solution by undoing the reordering of
insert hooks owned by the `stamp' and `match' modules.  The initial
reordering stems from d880a08f "Cement ordering of essential hook
members in ERC" and was intended to address the problem of timestamps
not being hidden in matched "fool" messages.  However, a more
comprehensive solution is being pursued as part of bug#64301.  The
following changes differ from those in a pure revert.

* erc/ERC-NEWS: Fix erroneous claim about relative hook ordering
pre-5.6.
* lisp/erc/erc-match.el (erc-match-mode, erc-match-enable): Lower hook
depth of `erc-match-message' in `erc-insert-modify-hook' from 60 to
50.
(erc-text-matched-hook): Retain cleaned-up portion of revised doc
string instead of reverting completely.
* lisp/erc/erc-stamp.el (erc-stamp-mode, erc-stamp-enable): Raise
depth of `erc-add-timestamp' in `erc-insert-modify-hook' and
`erc-send-modify-hook' from 50 to 60.
* test/lisp/erc/erc-scenarios-match.el
(erc-scenarios-match--stamp-left-current-nick
erc-scenarios-match--stamp-left-fools-invisible): Temporarily disable
the latter and fix expected hook ordering.
* test/lisp/erc/erc-tests.el (erc--essential-hook-ordering): Fix
expected order of default insert hooks.  (Bug#60936)