Fix unused import warnings (#2775)

Cargo.lock

@@ -1,3 +1,5 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
 [[package]]
 name = "adler32"
 version = "1.0.3"

core/src/core/pmmr/backend.rs

@@ -17,7 +17,6 @@ use croaring::Bitmap;
 use crate::core::hash::Hash;
 use crate::core::BlockHeader;
 use crate::ser::PMMRable;
-use std::path::Path;
 
 /// Storage backend for the MMR, just needs to be indexed by order of insertion.
 /// The PMMR itself does not need the Backend to be accurate on the existence

core/src/core/pmmr/pmmr.rs

@@ -22,7 +22,6 @@ use crate::core::merkle_proof::MerkleProof;
 use crate::core::pmmr::{Backend, ReadonlyPMMR};
 use crate::core::BlockHeader;
 use crate::ser::{PMMRIndexHashable, PMMRable};
-use std::path::Path;
 
 /// 64 bits all ones: 0b11111111...1
 const ALL_ONES: u64 = u64::MAX;