regexp/syntax: use the Regexp.Equal static method directly

regexp/syntax: use the Regexp.Equal static method directly

A follow-up for the recent https://go.dev/cl/573978.

Change-Id: I0e75ca0b37d9ef063bbdfb88d4d2e34647e0ee50
Reviewed-on: https://go-review.googlesource.com/c/go/+/574677
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>