internal: remove wycheproof tests

Equivalent coverage using newer Wycheproof vectors has been added to the
standard library, alongside the implementations under test.

In the case where the algorithm doesn't exist in the stdlib, but is
intended to be supported there one day (e.g. chacha20poly1305) we flip
the relationship and run the Wycheproof tests from the stdlib on the
vendored /x/crypto. As a result the legacy Wycheproof coverage in this
repo can be removed.

Change-Id: Ic07d559e42599b715906f5bcc39be10944a8dde4
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/758420
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>