doc: remove outdated information about interactive.singleKey

The Perl implementation of add --interactive was removed in commit [1].

Additionally, the interactive.singleKey setting is no longer silently
ignored. The internal implementation of ReadKey [2] displays a warning
if the platform is unsupported.

[1] 20b813d (add: remove "add.interactive.useBuiltin" & Perl "git add--interactive", 2023-02-06)
[2] a5e46e6 (terminal: add a new function to read a single keystroke, 2020-01-14)

Signed-off-by: Julio Bacellari <julio.bacel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: julio-b

Documentation/config/interactive.txt

@@ -4,9 +4,7 @@ interactive.singleKey::
 	Currently this is used by the `--patch` mode of
 	linkgit:git-add[1], linkgit:git-checkout[1],
 	linkgit:git-restore[1], linkgit:git-commit[1],
-	linkgit:git-reset[1], and linkgit:git-stash[1]. Note that this
-	setting is silently ignored if portable keystroke input
-	is not available; requires the Perl module Term::ReadKey.
+	linkgit:git-reset[1], and linkgit:git-stash[1].
 
 interactive.diffFilter::
 	When an interactive command (such as `git add --patch`) shows