[dev.boringcrypto] crypto/x509: remove VerifyOptions.IsBoring

[dev.boringcrypto] crypto/x509: remove VerifyOptions.IsBoring

This API was added only for BoringCrypto, never shipped in standard
Go. This API is also not compatible with the expected future evolution
of crypto/x509, as we move closer to host verifiers on macOS and Windows.

If we want to merge BoringCrypto into the main tree, it is best not to
have differing API. So instead of a hook set by crypto/tls, move the
actual check directly into crypto/x509, eliminating the need for
exposed API.

For #51940.

Change-Id: Ia2ae98c745de818d39501777014ea8166cab0b03
Reviewed-on: https://go-review.googlesource.com/c/go/+/395878
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>