[release-branch.go1.25] Revert "crypto/internal/fips140/subtle: add assembly implemen...

[release-branch.go1.25] Revert "crypto/internal/fips140/subtle: add assembly implementation of xorBytes for mipsx"

This reverts commit 343e486bfdbf9ca614d3e197afd79ad7ed5fef3e.

Reason for revert: doesn't handle unaligned accesses correctly.

Update #75790

Change-Id: I1d6210eeca9336f2ce311e99944cb270565563aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/709795
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
(cherry picked from commit cb81270113968408d7cc41c0b1530adb51dd8496)
Reviewed-on: https://go-review.googlesource.com/c/go/+/709758
Reviewed-by: David Chase <drchase@google.com>