Revert "Use number_of_parts for is_splittable"

It is too dangerous to rely on the admesh flag without inspecting the
admesh code line by line and a through test.

This reverts commit cd3cec3e45.

src/libslic3r/TriangleMesh.cpp

@@ -338,6 +338,19 @@ void TriangleMesh::rotate(double angle, Point* center)
     this->translate(c(0), c(1), 0);
 }
 
+/**
+ * Calculates whether or not the mesh is splittable.
+ */
+bool TriangleMesh::is_splittable() const
+{
+    std::vector<bool> visited;
+    find_unvisited_neighbors(visited);
+
+    // Try finding an unvisited facet. If there are none, the mesh is not splittable.
+    auto it = std::find(visited.begin(), visited.end(), false);
+    return it != visited.end();
+}
+
 /**
  * Visit all unvisited neighboring facets that are reachable from the first unvisited facet,
  * and return them.