[ModuleUtils] Clean up header file. [NFC]

Summary:
1. Remove unused forward declaration.
2. Remove unnecessary `\defgroup` in docygen documentation.

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70218

llvm/include/llvm/Transforms/Utils/ModuleUtils.h

@@ -109,23 +109,13 @@ void filterDeadComdatFunctions(
 /// unique identifier for this module, so we return the empty string.
 std::string getUniqueModuleId(Module *M);
 
-class TargetLibraryInfo;
 class CallInst;
 namespace VFABI {
-
-/// \defgroup Vector Function ABI (VABI) Module functions.
-///
-/// Utility functions for VFABI data that can modify the module.
-///
-/// @{
 /// Overwrite the Vector Function ABI variants attribute with the names provide
 /// in \p VariantMappings.
 void setVectorVariantNames(CallInst *CI,
                            const SmallVector<std::string, 8> &VariantMappings);
-
-/// @}
 } // End VFABI namespace
-
 } // End llvm namespace
 
 #endif //  LLVM_TRANSFORMS_UTILS_MODULEUTILS_H