Remove unused blake2b implementation

Author: Mark-Simulacrum

src/librustc_data_structures/blake2b.rs

@@ -57,7 +57,6 @@ pub mod small_vec;
 pub mod base_n;
 pub mod bitslice;
 pub mod bitvec;
-pub mod blake2b;
 pub mod graph;
 pub mod indexed_set;
 pub mod indexed_vec;