Removing unused GPBExceptionMessageKey

https://github.com/google/protobuf/issues/1618

objectivec/GPBMessage.h

@@ -55,13 +55,6 @@ typedef NS_ENUM(NSInteger, GPBMessageErrorCode) {
   GPBMessageErrorCodeMissingRequiredField = -101,
 };
 
-#ifdef DEBUG
-/// In DEBUG ONLY, an NSException is thrown when a parsed message doesn't
-/// contain required fields. This key allows you to retrieve the parsed message
-/// from the exception's @c userInfo dictionary.
-extern NSString *const GPBExceptionMessageKey;
-#endif  // DEBUG
-
 CF_EXTERN_C_END
 
 /// Base class for all of the generated message classes.

objectivec/GPBMessage.m

@@ -53,11 +53,6 @@
 NSString *const GPBMessageErrorDomain =
     GPBNSStringifySymbol(GPBMessageErrorDomain);
 
-#ifdef DEBUG
-NSString *const GPBExceptionMessageKey =
-    GPBNSStringifySymbol(GPBExceptionMessage);
-#endif  // DEBUG
-
 static NSString *const kGPBDataCoderKey = @"GPBData";
 
 //