Revert "Added #[derive(Default)] in some structs"

This reverts commit 209a624.

Author: manta1130

src/internal_scc.rs

@@ -35,7 +35,6 @@ struct _Edge {
 /// Reference:
 /// R. Tarjan,
 /// Depth-First Search and Linear Graph Algorithms
-#[derive(Default)]
 pub struct SccGraph {
     n: usize,
     edges: Vec<(usize, _Edge)>,

src/twosat.rs

@@ -1,6 +1,5 @@
 use crate::internal_scc;
 
-#[derive(Default)]
 pub struct TwoSat {
     n: usize,
     scc: internal_scc::SccGraph,