Burninate IDataPack (#864)

This doesn't break any extensions NOT using IDataPack, and we do not know of any that are.

* The extension storage utility of this interface has been broken for the last 9 months, with ISourceMod::CreateDataPack being disabled.
* The plugin interop utility of this interface (its stated purpose) has been broken for the last 11+ years, with ISourceMod::GetDataPackHandleType being disabled.

I imagine it only survived the first cleanup 11 years ago because CSS:DM was using it internally, which it has now been migrated away from.

Compiled all the included extensions without changes (API compat), and loaded extensions build pre-change without issue (ABI compat).

core/logic/CDataPack.cpp

@@ -46,7 +46,7 @@ CDataPack::~CDataPack()
 
 static ke::Vector<ke::AutoPtr<CDataPack>> sDataPackCache;
 
-IDataPack *CDataPack::New()
+CDataPack *CDataPack::New()
 {
   if (sDataPackCache.empty())
     return new CDataPack();
@@ -58,7 +58,7 @@ IDataPack *CDataPack::New()
 }
 
 void
-CDataPack::Free(IDataPack *pack)
+CDataPack::Free(CDataPack *pack)
 {
   sDataPackCache.append(static_cast<CDataPack *>(pack));
 }
@@ -187,11 +187,6 @@ const char *CDataPack::ReadString(size_t *len) const
 	return val.chars();
 }
 
-void *CDataPack::GetMemory() const
-{
-	return nullptr;
-}
-
 void *CDataPack::ReadMemory(size_t *size) const
 {
 	void *ptr = nullptr;