[std::vec] Rename .pop_opt() to .pop(), drop the old .pop() behavior

src/test/bench/msgsend-ring-mutex-arcs.rs

@@ -40,7 +40,7 @@ fn recv(p: &pipe) -> uint {
             while state.is_empty() {
                 cond.wait();
             }
-            state.pop()
+            state.pop().unwrap()
         })
     }
 }

src/test/bench/msgsend-ring-rw-arcs.rs

@@ -37,7 +37,7 @@ fn recv(p: &pipe) -> uint {
         while state.is_empty() {
             cond.wait();
         }
-        state.pop()
+        state.pop().unwrap()
     })
 }