curses: remove unnecessary special handling for dark gray

On terminals with at least 16 colors there should be no need for special
handling dark gray.

The curses code uses COLORS < 16, COLORS <= 16, COLORS > 16, or COLORS >= 16
at several places although I'm not sure if they are correct or which could
possibly be off-by-one errors.

But realistically in this case, we only need to distinguish between 8 color
terminals and terminals supporting more than 8 colors as this will mean the
terminal supports at least 256 colors.

win/curses/cursstat.c

@@ -1553,7 +1553,7 @@ curses_color_attr(int nh_color, int bg_color)
     if (!nh_color) {
 #ifdef USE_DARKGRAY
         if (iflags.wc2_darkgray) {
-            if (!can_change_color() || COLORS <= 16)
+            if (COLORS <= 16)
                 cattr |= A_BOLD;
         } else
 #endif