xbps-rindex: remove -c --clean mode and adapt to the new libxbps API.

- Removed -c --clean mode. It's not too useful and adds a considerable
  amount of gratuitous code. It takes almost the same time than
  adding all pkgs from scratch.

- When creating the repository data always add the meta plist at the
  2nd position in the archive, to optimize its access.

- Misc improvements.

bin/xbps-rindex/remove-obsoletes.c

@@ -119,10 +119,6 @@ remove_obsoletes(struct xbps_handle *xhp, const char *repodir)
 		}
 		return 0;
 	}
-	if ((repo->idx = xbps_repo_get_plist(repo, XBPS_REPOIDX)) == NULL) {
-		xbps_repo_close(repo);
-		return -1;
-	}
 	if (chdir(repodir) == -1) {
 		fprintf(stderr, "xbps-rindex: cannot chdir to %s: %s\n",
 		    repodir, strerror(errno));