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-by: Cherry Zhang <>
diff --git a/test/codegen/README b/test/codegen/README
index 1afefd9..f6877e7 100644
--- a/test/codegen/README
+++ b/test/codegen/README
@@ -30,12 +30,6 @@
 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