cmd/dist: eliminate registerSeqTest
cmd/dist: eliminate registerSeqTest

This is unused, and eliminating it lets us simplify the whole
registerTest mechanism.

Preparation for #37486.

Change-Id: Ia6221e48192cd17775a5d662bdb389d67a9265bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/448800
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go