Simplify option erc-merge-wrap-merge-indicator

Simplify option erc-merge-wrap-merge-indicator

* lisp/erc/erc-fill.el (erc-fill-wrap-merge-indicator): Reduce
offering of acceptable values by removing leading symbols and ditching
the `post' variant entirely.  The justification for the latter move
hinges on it not being well suited to next-gen requirements involving
the splicing and deletion of inserted messages.  Meeting these would
be overly burdensome and involve scanning the buffer in both
directions for every such operation.  This option is new in ERC 5.6,
which is yet unreleased.
(erc-fill--wrap-massage-legacy-indicator-type): New function to warn
of obsolete `erc-fill-wrap-merge-indicator' value and perform a
migration for the current session.
(erc-fill-wrap, erc-fill-wrap-enable): Preform preflight compat check
for obsolete `erc-fill-wrap-merge-indicator' value types.
(erc-fill-wrap-disable): Don't bother killing nonexistent variable
`erc-fill--wrap-merge-indicator-post'.
(erc-fill--wrap-merge-indicator-post): Remove unused variable.
(erc-fill--wrap-insert-merged-post): Remove unused function.
(erc-fill--wrap-insert-merged-pre): Adapt to simplified format for
option `erc-merge-wrap-merge-indicator'.
(erc-fill-wrap): Remove conditional dispatch because there is only one
path and only one indicator style.
(erc-fill--wrap-rejigger-region): Remove reference to nonexistent
variable `erc-fill--wrap-merge-indicator-post'.
* test/lisp/erc/erc-fill-tests.el
(erc-fill-wrap--merge-action/indicator-pre): Update format of value
for option `erc-fill-wrap-merge-indicator'.
(erc-fill-wrap--merge-action/indicator-post): Remove test focusing on
obsolete and unsupported `post' variant of option
`erc-fill-wrap-merge-indicator'.
(erc-fill--wrap-massage-legacy-indicator-type): New test.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-post-01.eld:
Delete file.  (Bug#60936)

(cherry picked from commit c572c30fb121008e5b248688ebe319dd85633c72)