GN: remove dead code in proto_library. (#3790)

`proto_library` users never set `use_protobuf_full`.
`//gn:protobuf_full` is only explicitly add do `deps` for specific build
targets.

This is a follow-up to 58f55df.

Author: KirillTim

gn/standalone/proto_library.gni

@@ -289,11 +289,7 @@ template("proto_library") {
         configs += [ ":$config_name" ]
       }
 
-      # Use protobuf_full only for tests.
-      if (defined(invoker.use_protobuf_full) &&
-          invoker.use_protobuf_full == true) {
-        deps = [ "//gn:protobuf_full" ]
-      } else if (generate_cc) {
+      if (generate_cc) {
         deps = [ "//gn:protobuf_lite" ]
       } else {
         deps = []