ui: Reimplement :suspend command for remote UIs.

- Remove suspend method from the UI protocol
- Handle `:suspend` by disconnecting the last channel that sent a request to
  nvim.

test/functional/ui/screen.lua

@@ -99,7 +99,6 @@ function Screen.new(width, height)
     _mouse_enabled = true,
     _bell = false,
     _visual_bell = false,
-    _suspended = true,
     _attrs = {},
     _cursor = {
       enabled = true, row = 1, col = 1
@@ -116,12 +115,10 @@ end
 
 function Screen:attach()
   request('ui_attach', self._width, self._height, true)
-  self._suspended = false
 end
 
 function Screen:detach()
   request('ui_detach')
-  self._suspended = true
 end
 
 function Screen:expect(expected, attr_ids)
@@ -286,10 +283,6 @@ function Screen:_handle_update_bg(bg)
   self._bg = bg
 end
 
-function Screen:_handle_suspend()
-  self._suspended = true
-end
-
 function Screen:_clear_block(top, lines, left, columns)
   for i = top, top + lines - 1 do
     self:_clear_row_section(i, left, left + columns - 1)