Remove grpc watch functionality (#2549)

This was added, but didn't see any use. For a large, complex chunk of
code we should have some users of it.

Remove all watch functionally from plugins, servers and packages.

Fixes: #2548

Signed-off-by: Miek Gieben <miek@miek.nl>

pb/Makefile

@@ -6,6 +6,8 @@
 all: dns.pb.go
 
 dns.pb.go: dns.proto
-	protoc --go_out=plugins=grpc:. dns.proto && \
-        sed -e s?golang.org/x/net/context?context? < dns.pb.go > dns.pb.go.tmp && \
-        mv dns.pb.go.tmp dns.pb.go
+	protoc --go_out=plugins=grpc:. dns.proto
+
+.PHONY: clean
+clean:
+	rm dns.pb.go