Remove subscription_t's clock.identifier field. (#125)

As noticed in
rust-lang/rust#65617 (comment),
the `clock.identifier` field in `subscription_t` isn't used for
anything. It came from CloudABI, where it appears to be a holdover from
an earlier API feature which is no longer present.

phases/ephemeral/witx/typenames.witx

@@ -552,8 +552,6 @@
 ;; The contents of a $subscription_t when type is `EVENTTYPE_CLOCK`.
 (typename $subscription_clock_t
  (struct
- ;; The user-defined unique identifier of the clock.
- (field $identifier $userdata_t)
  ;; The clock against which to compare the timestamp.
  (field $clock_id $clockid_t)
  ;; The absolute or relative timestamp.