Fix gRPC manager build script

[andreyvelich]

We are currently unable to re-run build.sh script to generate gRPC manager APIs.
I am getting this error:

:/apiprotos protoc-gen-doc --doc_opt=markdown,api.md -I /protobuf -I /apiprotos api.proto
[libprotobuf WARNING ../../../../../src/google/protobuf/compiler/parser.cc:562] No syntax specified for the proto file: google/api/annotations.proto. Please use 'syntax = "proto2";' or 'syntax = "proto3";' to specify a syntax version. (Defaulted to proto2 syntax.)
google/api/annotations.proto:1:1: Expected top-level statement (e.g. "message").
api.proto: Import "google/api/annotations.proto" was not found or had errors.

As I can see, the annotations and grpc-gateway_out scripts were added as part of this PR: #142.
We are not using REST manager more and that should be removed.
As well, I removed legacy Manager Swagger and Makefile.

/assign @gaocegege @johnugeorge

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreyvelich

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [andreyvelich]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[gaocegege]

LGTM 👍
/lgtm