Regression from c3dfe1e204.
Above commit put a dummy dragable button over the filename (well, to
allow dragging). Enabling drag ripples down to the button ending up in
the `BUTTON_STATE_HIGHLIGHT` state [which is still good of course].
However, `ui_do_but_EXIT` will then return `WM_UI_HANDLER_BREAK` which
then results in the filebrowser operator being skipped.
`ui_do_but_EXIT` already had a special case for file-browser drag button
[which would return `WM_UI_HANDLER_CONTINUE` instead]. Formerly this was
just the icon, now it is the icon and filename label. So in order to fix
this, remove the explicit check for the `but->imb` which will then
include the dragable label button in the exception for returning
`WM_UI_HANDLER_CONTINUE`.
Fixes#111645.
