UIManager: Don't lose track of the original rotation on reboot/poweroff (#9606)

And, actually, stop enforcing Portrait; Screensaver already handles
rotation as necessary depending on the exact screensaver mode.

Fix https://www.mobileread.com/forums/showthread.php?t=349522

frontend/ui/uimanager.lua

@@ -61,9 +61,7 @@ function UIManager:init()
     self.poweroff_action = function()
         self._entered_poweroff_stage = true
         logger.info("Powering off the device...")
-        Device.orig_rotation_mode = Device.screen:getRotationMode()
         self:broadcastEvent(Event:new("Close"))
-        Screen:setRotationMode(Screen.ORIENTATION_PORTRAIT)
         local Screensaver = require("ui/screensaver")
         Screensaver:setup("poweroff", _("Powered off"))
         Screensaver:show()
@@ -76,9 +74,7 @@ function UIManager:init()
     self.reboot_action = function()
         self._entered_poweroff_stage = true
         logger.info("Rebooting the device...")
-        Device.orig_rotation_mode = Device.screen:getRotationMode()
         self:broadcastEvent(Event:new("Close"))
-        Screen:setRotationMode(Screen.ORIENTATION_PORTRAIT)
         local Screensaver = require("ui/screensaver")
         Screensaver:setup("reboot", _("Rebooting…"))
         Screensaver:show()