`popover-content` class had no effect on emoji popover since
it was resetting `padding: 0` and background color was
already applied by tippy theme.
In short, removing `popover-content` had no visual changes.
Remove bootstrap-tooltip class `popover-title` from code.
It was only used in user_card_popover for showing user avatar,
so cleaned up properties that were duplicated or not required.