hash/crc32: remove single-use {castagnoli,ieee}ArchImpl bools

hash/crc32: remove single-use {castagnoli,ieee}ArchImpl bools

These are only used once right after being set in castagnoliInit and
ieeeInit, respectively.

Change-Id: Ifc5df1fe3040b97b1d252cc6d6d28d8fbc132fa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/423854
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>