Remove tests/lib.rs to avoid running tests twice.

The 'cargo test' command simply compiles each 'tests/*.rs' and runs the
enclosed tests. The 'lib.rs' source would get run as an individual test.

cranelift/src/tools/tests/lib.rs

@@ -1,2 +0,0 @@
-pub mod cfg_traversal;
-pub mod dominator_tree;