Revert "Enable syncing of redraws to monitor refresh rate"

This reverts commit 6a51096304.
Turns out that dynamically changing the swap interval causes bad flicker
when rendering on X11 in a fullscreen window (or more generally, when
only a single X11 window is visible).

kitty/screen.h

@@ -51,12 +51,6 @@ typedef struct {
 } SavemodesBuffer;
 
 
-typedef struct {
-    bool large_change;
-    unsigned int chars_written;
-} RenderActivitity;
-
-
 typedef struct {
     PyObject_HEAD
 
@@ -90,7 +84,6 @@ typedef struct {
     pthread_mutex_t read_buf_lock, write_buf_lock;
 
     CursorRenderInfo cursor_render_info;
-    RenderActivitity render_activity;
 
 } Screen;