Remove ConcurrentHashMap.

ConcurrentHashMap is unused and has a lot of uses of transmute_mut, so
it's best to remove it.
auto merge of #2509 : zwarich/servo/remove-concurrent-map, r=pcwalton

ConcurrentHashMap is unused and has a lot of uses of transmute_mut, so
it's best to remove it.
auto merge of #2490 : saneyuki/servo/js, r=jdm

This only change to make `JS<T> ` to a POD type about #1854, except using `Cell`/`RefCell` for interior mutability.

Fix #1854

@jdm r?
"JS<T>::from_raw" should accept "*T" instead of "*mut T"