stash: remove documentation for `stash.useBuiltin`

In 8a2cd3f512 (stash: remove the stash.useBuiltin setting, 2020-03-03),
we removed the setting, and for a couple of major versions, we still
documented the setting, telling users that it is gone.

We can now safely remove even the documentation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/stash.txt

@@ -1,10 +1,3 @@
-stash.useBuiltin::
-	Unused configuration variable.  Used in Git versions 2.22 to
-	2.26 as an escape hatch to enable the legacy shellscript
-	implementation of stash.  Now the built-in rewrite of it in C
-	is always used. Setting this will emit a warning, to alert any
-	remaining users that setting this now does nothing.
-
 stash.showIncludeUntracked::
 	If this is set to true, the `git stash show` command will show
 	the untracked files of a stash entry.  Defaults to false. See