Do not use string "None" for different avatar options

There are some issues on the edit avatar page in UCP:

• Avatar type: the option "None" is unnecessary since you can delete the avatar with "Delete image"
• When using a gallery avatar, the category "None" is also unnecessary (see also issue #12457)
• When "None" is kept, avatar type and category shouldn't use the same language var (NO_AVATAR_CATEGORY) since avatar type has not the same gender as category in some languages
• Also the main category shouldn't use MAIN since you need a different translation in some cases