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.hpp

@@ -68,6 +68,7 @@ public:
     size_t facets_count() const { return this->stl.stats.number_of_facets; }
     bool   empty() const { return this->facets_count() == 0; }
 
+    bool is_splittable() const;
     std::deque<uint32_t> find_unvisited_neighbors(std::vector<bool> &facet_visited) const;
 
     stl_file stl;