Revert "salsa20: add loong64 SIMD implementation"

This reverts CL 663375.

Reason for revert: Does not build on 1.23 or 1.24

For golang/go#73354.

Change-Id: I251d598423b83c01cc2e04ddf6f49ae14095fa7c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/665535
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: abner chenc <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

salsa20/salsa/salsa20_test.go renamed to salsa20/salsa/salsa20_amd64_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (amd64 || loong64) && !purego && gc
+//go:build amd64 && !purego && gc
 
 package salsa