remove disable_char_avail_for_testing()

test_cursor_func.vim hangs at the call to
disable_char_avail_for_testing(). The test does not actually need this
function (and it correctly fails if the fix from 7.4.1300 is reverted).
Given that disable_char_avail_for_testing is a gigantic hack, if we can
avoid it let's do so.

src/nvim/getchar.c

@@ -1562,11 +1562,6 @@ int char_avail(void)
 {
   int retval;
 
-  // When disable_char_avail_for_testing(1) was called pretend
-  // there is no typeahead
-  if (disable_char_avail_for_testing) {
-    return false;
-  }
   no_mapping++;
   retval = vpeekc();
   --no_mapping;