Revert "Add bold off SGR"

This reverts commit 6cb6d61525.
This wasn't a useful thing after all.

st.c

@@ -1402,9 +1402,6 @@ tsetattr(int *attr, int l)
 		case 9:
 			term.c.attr.mode |= ATTR_STRUCK;
 			break;
-		case 21:
-			term.c.attr.mode &= ~ATTR_BOLD;
-			break;
 		case 22:
 			term.c.attr.mode &= ~(ATTR_BOLD | ATTR_FAINT);
 			break;