crypto/internal/bigmod: drop math/big dependency

crypto/internal/bigmod: drop math/big dependency

If when the dust settles the Bytes and SetBytes round-trip is visible in
profiles (only plausible in RSA), then we can add a SetBits method like
in CL 511375.

Change-Id: I3e6677e849d7a3786fa7297437b119a47715225f
Reviewed-on: https://go-review.googlesource.com/c/go/+/628675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>