Remove unused extern declarations (#13159)

Author: parjong

src/vm/prestub.cpp

@@ -60,14 +60,6 @@ EXTERN_C void DeleteFromPitchingCandidate(MethodDesc* pMD);
 EXTERN_C void MarkMethodNotPitchingCandidate(MethodDesc* pMD);
 #endif
 
-EXTERN_C void STDCALL ThePreStub();
-
-#if defined(HAS_COMPACT_ENTRYPOINTS) && defined (_TARGET_ARM_)
-
-EXTERN_C void STDCALL ThePreStubCompactARM();
-
-#endif // defined(HAS_COMPACT_ENTRYPOINTS) && defined (_TARGET_ARM_)
-
 EXTERN_C void STDCALL ThePreStubPatch();
 
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