all: drop -mod=vendor in integration test invocations

Drop -mod=vendor because it's not needed by now. There are multiple
reasons for this: 1) integration_test.go has no external dependencies,
2) there's no checked-in vendor directory, 3) CL 544855 deleted the
last 'go mod vendor' invocation in code, and 4) now that the go.mod
has a go directive at 1.14+ (CL 547337), automatic vendoring¹ is on.

For golang/go#64066.

¹ https://go.dev/doc/go1.14#vendor, https://go.dev/ref/mod#vendoring

Change-Id: If8f3ea85d560b2796a2ccdb4ae7dbc38f0760982
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/548115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>

regenerate.bash

@@ -4,5 +4,5 @@
 # license that can be found in the LICENSE file.
 
 cd "$(git rev-parse --show-toplevel)"
-go test -v -mod=vendor -timeout=60m -count=1 integration_test.go "$@" -regenerate
+go test -v -timeout=60m -count=1 integration_test.go "$@" -regenerate
 exit $?

release.bash

@@ -79,7 +79,7 @@ fi
 git commit -a -m "all: release $(version_string)"
 
 # Build release binaries.
-go test -mod=vendor -timeout=60m -count=1 integration_test.go "$@" -buildRelease
+go test -timeout=60m -count=1 integration_test.go "$@" -buildRelease
 
 # Create commit to start development after release.
 VERSION_PRERELEASE="${VERSION_PRERELEASE}.devel" # append ".devel"

test.bash

@@ -4,5 +4,5 @@
 # license that can be found in the LICENSE file.
 
 cd "$(git rev-parse --show-toplevel)"
-go test -v -mod=vendor -timeout=60m -count=1 integration_test.go -failfast "$@"
+go test -v -timeout=60m -count=1 integration_test.go -failfast "$@"
 exit $?