CLEANUP: ebmbtree: Replace always-taken elseif by else

`diff` is guaranteed to be less than 0, because the `if` handles the `>= 0`
case.

Found using GitHub's CodeQL scan in HAProxy's codebase.
diff --git a/include/import/ebmbtree.h b/include/import/ebmbtree.h
index f99c16b..1a45426 100644
--- a/include/import/ebmbtree.h
+++ b/include/import/ebmbtree.h
@@ -358,7 +358,7 @@
 		new->node.leaf_p = new_rght;
 		*up_ptr = new_left;
 	}
-	else if (diff < 0) {
+	else {
 		new->node.branches.b[EB_LEFT] = new_leaf;
 		new->node.branches.b[EB_RGHT] = troot;
 		new->node.leaf_p = new_left;