Delete "Utility" from .desktop file

[SiqingYu]

I know this sounds like a joke. While I was showering, I realized that WeChat isn't an application in the "Utility" category. "Utilities" are accessory applications like archiver, calculator, clock. For example, Telegram doesn't have "Utility" in its category: Categories=Network;InstantMessaging;Qt; I also noticed that there is a difference between "Chat" and "Instant Messaging." IMO WeChat is an instant messaging app instead of a chat app. I am sorry for I made that negligent pull request without even thinking about it.

Reference:

• Desktop Menu Specification - freedesktop.org
• https://github.com/telegramdesktop/tdesktop/blob/dev/lib/xdg/telegramdesktop.desktop
• Differences Between Chat and Instant Messaging - Lifewire
• https://en.wikipedia.org/wiki/WeChat (Notice that the "Type" is "Instant messaging client")