Documentation/git-clean.txt: don't mention deletion of .git/modules/*

The latter half of this sentence, the removal of the submodules, was
never done with (or without) double -f back when it was written, and
we still do not do so.

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -37,9 +37,7 @@ OPTIONS
 	to false, 'git clean' will refuse to delete files or directories
 	unless given -f, -n or -i. Git will refuse to delete directories
 	with .git sub directory or file unless a second -f
-	is given. This affects also git submodules where the storage area
-	of the removed submodule under .git/modules/ is not removed until
-	-f is given twice.
+	is given.
 
 -i::
 --interactive::