Remove redundant imports to fix CI (#1876)

exercises/practice/poker/.meta/example.rs

@@ -1,5 +1,5 @@
+use std::cmp::Ordering;
 use std::fmt;
-use std::{cmp::Ordering, convert::TryFrom};
 
 use counter::Counter;
 

exercises/practice/simple-linked-list/.meta/example.rs

@@ -1,5 +1,3 @@
-use std::iter::FromIterator;
-
 pub struct SimpleLinkedList<T> {
     head: Option<Box<Node<T>>>,
     len: usize,

exercises/practice/simple-linked-list/src/lib.rs

@@ -1,5 +1,3 @@
-use std::iter::FromIterator;
-
 pub struct SimpleLinkedList<T> {
     // Delete this field
     // dummy is needed to avoid unused parameter error during compilation