Revert "crypto/internal/fips140/subtle: add assembly implementation of xorBytes for mips64x"

This reverts commit 49d6777.

Reason for revert: doesn't handle unaligned accesses correctly

Fixes golang#74998

Change-Id: Ia272245a6a2a91b305d411207430bad660ee355b
Reviewed-on: https://go-review.googlesource.com/c/go/+/709757
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: randall77

src/crypto/internal/fips140/subtle/xor_asm.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 || arm64 || mips64 || mips64le || ppc64 || ppc64le || riscv64) && !purego
+//go:build (amd64 || arm64 || ppc64 || ppc64le || riscv64) && !purego
 
 package subtle
 

src/crypto/internal/fips140/subtle/xor_generic.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 && !arm64 && !loong64 && !mips64 && !mips64le && !ppc64 && !ppc64le && !riscv64) || purego
+//go:build (!amd64 && !arm64 && !loong64 && !ppc64 && !ppc64le && !riscv64) || purego
 
 package subtle