cmd/go: don't mention -mod=release

The -mod=release flag is not supported, so this appears to be a
documentation mistake.

Fixes #27354.

Change-Id: I895e8d5b4918adcb1f605361773173f312fa7b65
GitHub-Last-Rev: 42bfe0c11e38c90e76887771654ea81af98d50ec
GitHub-Pull-Request: golang/go#27358
Reviewed-on: https://go-review.googlesource.com/132116
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -144,7 +144,7 @@
 // 		link against shared libraries previously created with
 // 		-buildmode=shared.
 // 	-mod mode
-// 		module download mode to use: readonly, release, or vendor.
+// 		module download mode to use: readonly or vendor.
 // 		See 'go help modules' for more.
 // 	-pkgdir dir
 // 		install and load all packages from dir instead of the usual locations.

src/cmd/go/internal/work/build.go

@@ -99,7 +99,7 @@ and test commands:
 		link against shared libraries previously created with
 		-buildmode=shared.
 	-mod mode
-		module download mode to use: readonly, release, or vendor.
+		module download mode to use: readonly or vendor.
 		See 'go help modules' for more.
 	-pkgdir dir
 		install and load all packages from dir instead of the usual locations.