[netstack3] Remove `PortAlloc`

ICMP and UDP sockets now use the Algorithm 1 from RFC 6056 to fully
randomize. This will make it harder for an adversarial to try to
track a user.

A test was removed because a property being tested no longer holds.

Fixed: 42083786
Bug: 42182649
Change-Id: I3c8b9170eccf71b2804ae6bfff6b1d385c884d74
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1000514
Reviewed-by: Bruno Dal Bo <brunodalbo@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Zeling Feng <zeling@google.com>