JS_SetContextPrivate and friends are being removed

In the next release of SpiderMonkey, each thread that wants to use JS must
have a single JSRuntime and a single JSContext. The two concepts are being
unified. Work is ongoing; this week, <
https://bugzilla.mozilla.org/show_bug.cgi?id=1281529>.

As a result, we're removing these API functions:
  JS_GetContextPrivate
  JS_SetContextPrivate
  JS_GetSecondContextPrivate
  JS_SetSecondContextPrivate

Rationale: Once you switch to a single JSRuntime and a single JSContext
per-thread, these functions are useless. You can easily switch to
thread-local or global variables, which are much nicer: they have names and
types, they can be any size, and you can have as many as you like.

See <https://bugzilla.mozilla.org/show_bug.cgi?id=1281582>.

Cheers,
-j