Remove duplicate grpc_census.cc build

BUILD

@@ -2201,7 +2201,6 @@ grpc_cc_library(
 grpc_cc_library(
     name = "grpc_opencensus_plugin",
     srcs = [
-        "src/core/ext/filters/census/grpc_context.cc",
         "src/cpp/ext/filters/census/channel_filter.cc",
         "src/cpp/ext/filters/census/client_filter.cc",
         "src/cpp/ext/filters/census/context.cc",