ui: disable clearing almost everywhere

Avoid clearing the screen in most situations. NOT_VALID should be
equivalent to CLEAR unless some external force messed up the terminal,
for these situations <c-l> and :mode will still clear the screen.

Also eliminate some obsolete code in screen.c, that dealt with that in
vim drawing window 1 can mess up window 2, but this never happens in
nvim.

But what about slow terminals? There is two common meanings in which
a terminal is said to be "slow":

Most commonly (and in the sense of vim:s nottyfast) it means low
bandwidth for sending bytes from nvim to the terminal. If the screen is
very similar before and after the update_screen(CLEAR) this change
should reduce bandwidth. If the screen is quite different, but there is
no new regions of contiguous whitespace, clearing doesn't reduce
bandwidth significantly. If the new screen contains a lot of whitespace,
it will depend of if vsplits are used or not: as long as there is no
vsplits, ce is used to cheaply clear the rest of the line, so
full-screen clear is not needed to reduce bandwith. However a left
vsplit currently needs to be padded with whitespace all the way to the
separator. It is possible ec (clear N chars) can be used to reduce
bandwidth here if this is a problem. (All of this assumes that one
doesn't set Normal guibg=... on a non-BCE terminal, if you do you are
doomed regardless of this change).

Slow can also mean that drawing pixels on the screen is slow. E-ink
screens is a recent example. Avoiding clearing and redrawing the
unchanged part of the screen will always improve performance in these
cases.

src/nvim/ex_getln.c

@@ -214,8 +214,6 @@ static int hislen = 0;                  /* actual length of history tables */
 /// user interrupting highlight function to not interrupt command-line.
 static bool getln_interrupted_highlight = false;
 
-static bool need_cursor_update = false;
-
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_getln.c.generated.h"
@@ -3020,8 +3018,6 @@ void cmdline_screen_cleared(void)
     }
     line = line->prev_ccline;
   }
-
-  need_cursor_update = true;
 }
 
 /// called by ui_flush, do what redraws neccessary to keep cmdline updated.
@@ -3481,10 +3477,7 @@ static void cursorcmd(void)
     if (ccline.redraw_state < kCmdRedrawPos) {
       ccline.redraw_state = kCmdRedrawPos;
     }
-    if (need_cursor_update) {
-      need_cursor_update = false;
-      setcursor();
-    }
+    setcursor();
     return;
   }