cmd/gofmt: remove stale documentation for the -G flag

This documentation remained from the original dev.typeparams merge. This
flag no longer exists.

Change-Id: Ic9a82071c512614dc1382780d69ef13253fca21d
Reviewed-on: https://go-review.googlesource.com/c/go/+/329792
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/src/cmd/gofmt/doc.go b/src/cmd/gofmt/doc.go
index 68476e7..e340665 100644
--- a/src/cmd/gofmt/doc.go
+++ b/src/cmd/gofmt/doc.go
@@ -26,9 +26,6 @@
 		Do not print reformatted sources to standard output.
 		If a file's formatting is different from gofmt's, print its name
 		to standard output.
-	-G
-		Allow generic code, using type parameters.
-		See golang.org/issues/43651 for more information.
 	-r rule
 		Apply the rewrite rule to the source before reformatting.
 	-s