net: Use DynamicUsage(m_type) in CSerializedNetMsg::GetMemoryUsage

Now that memusage correctly computes the dynamic size of a string, there
is no need for special handling here.

src/net.cpp

@@ -121,10 +121,7 @@ std::string strSubVersion;
 
 size_t CSerializedNetMsg::GetMemoryUsage() const noexcept
 {
-    // Don't count the dynamic memory used for the m_type string, by assuming it fits in the
+    return sizeof(*this) + memusage::DynamicUsage(m_type) + memusage::DynamicUsage(data);
-    // "small string" optimization area (which stores data inside the object itself, up to some
-    // size; 15 bytes in modern libstdc++).
-    return sizeof(*this) + memusage::DynamicUsage(data);
 }
 
 void CConnman::AddAddrFetch(const std::string& strDest)