remove test proto for field presence

csharp/protos/google/protobuf/field_presence_test.proto

-syntax = "proto3";
-
-package field_presence_test;
-
-option java_package = "com.google.protobuf";
-option java_outer_classname = "FieldPresenceTestProto";
-option java_generate_equals_and_hash = true;
-
-message TestAllTypes {
-  enum NestedEnum {
-    FOO = 0;
-    BAR = 1;
-    BAZ = 2;
-  }
-  message NestedMessage {
-    optional int32 value = 1;
-  }
-
-  optional int32 optional_int32 = 1;
-  optional string optional_string = 2;
-  optional bytes optional_bytes = 3;
-  optional NestedEnum optional_nested_enum = 4;
-  optional NestedMessage optional_nested_message = 5;
-}