Also remove paragraph duplicated during the merge v2->master

Fixes #1751

When option module=... is used, protoc-gen-grpc-gateway removes the module
prefix a first time, then uses protogen, which tries to remove the prefix
a second time.

Fixes #1753

* Refactor registry and generator to use protogen

* fix tests in genopenapi/template_test.go

* Use FieldMask from protobuf/types/known/fieldmaskpb

* Revert "Use FieldMask from protobuf/types/known/fieldmaskpb"

This reverts commit 953a114.

* Revert usage of google.golang.org/protobuf/types/known/fieldmaskpb

* Fix internal/gengateway/BUILD.bazel

* Adapt usage instructions to use protoc-gen-go-grpc

This reflect recent changes in https://grpc.io/docs/languages/go/quickstart/
to use new protoc-gen-go-grpc.

Also remove a duplicated paragraph.

* Readme: use new protoc-gen-go

Replace github.com/golang/protobuf/protoc-gen-go by
google.golang.org/protobuf/cmd/protoc-gen-go

* Update other instructions to use protoc-gen-go-grpc

* Use --x-opt in instructions

* Update docs/_docs/usegotemplates.md

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

* Use go get instead of go get -u

Co-authored-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>

* return registry from gengateway.New

retain parsing order

comment

* update test after merge

* add example to abe

* bad merge

* cut and paste err

* simple change

* simplify

* outdated

* goimports