Revert "test/codegen: document -all_codegen option in README"

This reverts CL 192101.

Reason for revert: The same paragraph was added 2 weeks ago
(look a few lines above)

Change-Id: I05efb2631d7b4966f66493f178f2a649c715a3cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/195637Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/codegen/README

@@ -30,12 +30,6 @@ transformation, it can be useful to first run the test harness with a
 toolchain from a released Go version (and verify that the new tests
 fail), and then re-runnig the tests using the devel toolchain.
 
-By default, only checks that are relevant to the current GOOS/GOARCH
-are run. Checks for all platforms can be enabled by specifiying the
--all_codegen option, as
-
-  $ ../bin/go run run.go -all_codegen codegen
-
 
 - Regexps comments syntax