Documentation/blame-options: don't list date formats

This list is already incomplete (missing "raw") and we're about to add
new formats.  Remove it and refer to the canonical documentation in
git-log(1).

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -63,11 +63,10 @@ include::line-range-format.txt[]
 	`-` to make the command read from the standard input).
 
 --date <format>::
-	The value is one of the following alternatives:
-	{relative,local,default,iso,rfc,short}. If --date is not
+	Specifies the format used to output dates. If --date is not
 	provided, the value of the blame.date config variable is
 	used. If the blame.date config variable is also not set, the
-	iso format is used. For more information, See the discussion
+	iso format is used. For supported values, see the discussion
 	of the --date option at linkgit:git-log[1].
 
 -M|<num>|::