Remove GRXWriter wrappers that are now unnecessary

With GRXWriter a subclass of all writers, we can map the requestsWriter directly
without a dumb wrapper, and return GRXImmediateWriter objects where GRXWriter is
expected.

src/objective-c/ProtoRPC/ProtoRPC.m

@@ -35,7 +35,6 @@
 
 #import <GPBProtocolBuffers.h>
 #import <RxLibrary/GRXWriteable.h>
-#import <RxLibrary/GRXForwardingWriter.h>
 #import <RxLibrary/GRXWriter+Transformations.h>
 
 @implementation ProtoRPC {
@@ -65,8 +64,7 @@
                 format:@"A protobuf class to parse the responses must be provided."];
   }
   // A writer that serializes the proto messages to send.
-  id<GRXWriter> bytesWriter =
+  GRXWriter *bytesWriter = [requestsWriter map:^id(GPBMessage *proto) {
-      [[[GRXForwardingWriter alloc] initWithWriter:requestsWriter] map:^id(GPBMessage *proto) {
     // TODO(jcanizales): Fail with an understandable error message if the requestsWriter isn't
     // sending GPBMessages.
     return [proto data];

src/objective-c/RxLibrary/GRXWriter+Immediate.m

@@ -38,27 +38,27 @@
 @implementation GRXWriter (Immediate)
 
 + (instancetype)writerWithEnumerator:(NSEnumerator *)enumerator {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter writerWithEnumerator:enumerator]];
+  return [GRXImmediateWriter writerWithEnumerator:enumerator];
 }
 
 + (instancetype)writerWithValueSupplier:(id (^)())block {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter writerWithValueSupplier:block]];
+  return [GRXImmediateWriter writerWithValueSupplier:block];
 }
 
 + (instancetype)writerWithContainer:(id<NSFastEnumeration>)container {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter writerWithContainer:container]];
+  return [GRXImmediateWriter writerWithContainer:container];
 }
 
 + (instancetype)writerWithValue:(id)value {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter writerWithValue:value]];
+  return [GRXImmediateWriter writerWithValue:value];
 }
 
 + (instancetype)writerWithError:(NSError *)error {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter writerWithError:error]];
+  return [GRXImmediateWriter writerWithError:error];
 }
 
 + (instancetype)emptyWriter {
-  return [[self alloc] initWithWriter:[GRXImmediateWriter emptyWriter]];
+  return [GRXImmediateWriter emptyWriter];
 }
 
 @end