* Remove show prop and related logic from PopoverBar component

* Remove reference to this prop from ViewImageModal component

* Update PopoverBar's Jest snapshots

As noted in issue mattermost#12018, this prop was required earlier to
conditionally show/hide the component on mouse enter/leave on modal.

But now the component set to permanently show directly via CSS,
which makes the prop unnecessary and obsolete.

* Rename the prop as `showCloseBtn`

* Update its references in unit tests

* Update Jest snapshops for ViewImageModal

Now that the PopoverBar's `show` prop has been removed, the showFooter
state needs to be renamed as `showCloseBtn` to better reflect its
renewed purpose.