SFMLHelper: Simplify 64-bit packet reading function and remove 64-bit write function

Now that SFML's packet class can properly handle 64-bit values, we don't
need a helper function just to write values to the packets.

Source/Core/Common/SFMLHelper.h

@@ -19,5 +19,4 @@ struct BigEndianValue;
 u16 PacketReadU16(sf::Packet& packet);
 u32 PacketReadU32(sf::Packet& packet);
 u64 PacketReadU64(sf::Packet& packet);
-void PacketWriteU64(sf::Packet& packet, u64 value);
 }  // namespace Common