mobile: remove grpc-gateway import

Because we compile the REST code from the rest-annotations.yaml and no
longer import the annotations in the proto files, we don't need to
specify the custom import path anymore.

mobile/gen_bindings.sh

@@ -47,7 +47,6 @@ for file in $PROTOS; do
   echo "Generating mobile protos from ${file}"
 
   protoc -I/usr/local/include -I. \
-         -I$GOPATH/src/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis \
          --plugin=protoc-gen-custom=$falafel\
          --custom_out=./build \
          --custom_opt="$opts" \
@@ -77,7 +76,6 @@ do
     echo "Generating mobile protos from ${file}, with build tag ${tag}"
 
     protoc -I/usr/local/include -I. \
-           -I$GOPATH/src/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis \
            -I../lnrpc \
            --plugin=protoc-gen-custom=$falafel \
            --custom_out=./build \