remove modvendor tool

Not needed anymore since we upgraded go-ethereum, including this fix:
ethereum/go-ethereum#20232

The vendor changes are from running `go mod vendor`, which now
includes the secp256k1.h file previously not vendored due to the above
bug.

Makefile

@@ -26,7 +26,6 @@ envinit:
 	GO111MODULE=off go get -u github.com/stretchr/testify # needed for mockery
 	GO111MODULE=on go get -u github.com/vektra/mockery/...
 	GO111MODULE=off go get -u github.com/matryer/moq
-	GO111MODULE=off go get -u github.com/goware/modvendor
 	GO111MODULE=off go get golang.org/x/tools/cmd/goimports
 	GO111MODULE=off go get -u golang.org/x/mobile/cmd/gomobile
 	GO111MODULE=off gomobile init
@@ -96,4 +95,3 @@ locize-fix:
 	locize format ${WEBROOT}/src/locales --format json
 go-vendor:
 	go mod vendor
-	modvendor -copy="**/*.c **/*.h **/*.proto" -v