rollup merge of #20976: jbcrail/rm-unused-import

The BorrowFrom trait was unused by the tests.

src/libcollections/btree/map.rs

@@ -1422,7 +1422,6 @@ pub fn entry<'a>(&'a mut self, mut key: K) -> Entry<'a, K, V> {
 #[cfg(test)]
 mod test {
     use prelude::*;
-    use std::borrow::BorrowFrom;
 
     use super::{BTreeMap, Occupied, Vacant};