Revert "chacha20: add loong64 SIMD implementation"

This reverts CL 636257.

Reason for revert: Does not build on 1.23 or 1.24

For golang/go#73354.

Change-Id: Ie28d5c45930a8eacd5cabed64390ef674b3fb446
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/665536
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: prattmic