MetadataLoader.h - remove unnecessary Error forward declaration. NFC.

We need to include Error.h already

Author: RKSimon

llvm/lib/Bitcode/Reader/MetadataLoader.h

@@ -23,7 +23,6 @@ namespace llvm {
 class BitcodeReaderValueList;
 class BitstreamCursor;
 class DISubprogram;
-class Error;
 class Function;
 class Instruction;
 class Metadata;