Revert "imgui: Add get_window_width, get_window_height helper functions"

This reverts commit 0d10358942409cf8fad6fcb730dbedfbd7149df7.

`get_window_size` already exists and it's very easy to get just one
of the dimentions. As a result, delete get_window_width and
get_window_height.

src/lib.rs

@@ -392,18 +392,6 @@ impl<'ui> Ui<'ui> {
         }
         (out.x, out.y)
     }
-    /// Get current window's width in pixels
-    pub fn get_window_width(&self) -> f32 {
-        unsafe {
-            sys::igGetWindowWidth()
-        }
-    }
-    /// Get current window's height in pixels
-    pub fn get_window_height(&self) -> f32 {
-        unsafe {
-            sys::igGetWindowHeight()
-        }
-    }
 }
 
 // Layout