patch 8.1.1161: unreachable code

Problem:    Unreachable code.
Solution:   Remove condition that will never be true.  Add tests for all ANSI
            colors.
diff --git a/src/terminal.c b/src/terminal.c
index c7dc23f..c0777b2 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -2432,10 +2432,7 @@
 
     if (color->ansi_index != VTERM_ANSI_INDEX_NONE)
     {
-	/* First 16 colors and default: use the ANSI index, because these
-	 * colors can be redefined, we use the RGB values. */
-	if (t_colors > 256)
-	    return color->ansi_index;
+	// The first 16 colors and default: use the ANSI index.
 	switch (color->ansi_index)
 	{
 	    case  0: return 0;