Cleanup the forcing of portrait mode

1. Move the setting of G_width/G_height inside the function
Screen:setRotationMode()
2. Don't call self:clearCache() twice on closing the document (once is enough).
3. Get rid of UniReader:setRotationMode() method because
Screen:setRotationMode() is enough, as there is nothing reader- or
document-specific in this function.

reader.lua

@@ -122,7 +122,6 @@ Screen.native_rotation_mode = Screen.cur_rotation_mode
 
 -- force portrait mode
 Screen:setRotationMode(0)
-G_width, G_height = fb:getSize()
 
 -- set up reader's setting: font
 G_reader_settings = DocSettings:open(".reader")

screen.lua

@@ -58,6 +58,7 @@ function Screen:setRotationMode(mode)
 	fb:setOrientation(self.cur_rotation_mode)
 	fb:close()
 	fb = einkfb.open("/dev/fb0")
+	G_width, G_height = fb:getSize()
 end
 
 -- @orien: 1 for clockwise rotate, -1 for anti-clockwise

unireader.lua

@@ -1893,14 +1893,6 @@ function UniReader:screenRotate(orien)
 	self:clearCache()
 end
 
-function UniReader:setRotationMode(mode)
-	Screen:setRotationMode(mode)
-	-- update global width and height variable
-	G_width, G_height = fb:getSize()
-	self:clearCache()
-end
-
-
 function UniReader:cleanUpTocTitle(title)
 	return (title:gsub("\13", ""))
 end
@@ -2512,7 +2504,7 @@ function UniReader:inputLoop()
 	self.toc_cview = nil
 	self.toc_curidx_to_x = nil
 	self.show_overlap = 0
-	self:setRotationMode(0)
+	Screen:setRotationMode(0)
 	self:setDefaults()
 	if self.doc ~= nil then
 		self.doc:close()