tui: Disable interference in guicursor by higher layers.

Ironically, higher layers trying to be "smart" about the terminal type
but not actually being very smart at all, makes it more difficult rather
than less to correct the TUI layer.

Note that this orphans the os_term_is_nice() function and down the road,
presuming that we do not have to revert this, that function can be removed.
It incorporates knowledge of terminal types and behaviours in the wrong place.

src/nvim/option.c

@@ -969,10 +969,12 @@ void set_init_2(bool headless)
     p_window = Rows - 1;
   }
   set_number_default("window", Rows - 1);
+#if 0 // This bodges around problems that should properly be fixed in the TUI layer.
   if (!headless && !os_term_is_nice()) {
     set_string_option_direct((char_u *)"guicursor", -1, (char_u *)"",
                              OPT_GLOBAL, SID_NONE);
   }
+#endif
   parse_shape_opt(SHAPE_CURSOR);   // set cursor shapes from 'guicursor'
   (void)parse_printoptions();      // parse 'printoptions' default value
 }