Remove "buf mod update"

Do not upgrade locked dependencies during build.

This is only necessary to initialize the buf.lock file, once the
file exists, a regular "buf build" appears to pull them in.

Change-Id: I86d7734f8d6897bc30a6c9edd220eb51628be50c

buf.lock

@@ -12,6 +12,6 @@ deps:
     owner: grpc-ecosystem
     repository: grpc-gateway
     branch: main
-    commit: d19475fa22444a289c46af009acce62c
-    digest: b1-_zhDPyr_Ctc1QRAKuad6_0xvoyPd6QaB22ldm9gzS0Q=
-    create_time: 2021-04-26T15:19:26.742789Z
+    commit: 3d91372e5af6451c8af137f3b5594ac6
+    digest: b1-Iy4ANYcU1Z6T0LEswISKGoxqT-UJNNHWlJS6ZFhno4o=
+    create_time: 2021-08-01T01:14:29.490277Z

generate-protos.sh

@@ -11,6 +11,5 @@ rm -rf third_party/googleapis
 
 rm -rf bridge/pkg/proto
 
-tools/bin/buf mod update
 tools/bin/buf lint
 tools/bin/buf generate