CRYPTO: Remove the check for built-in methods in the export_to function

That check was seen as necessary at the time, but other changes have
been made since, so we now have better control on when we're handling
legacy structures and methods, making it safe to run the export_to
function on keys with foreign methods.

The basic message is that foreign methods must set key structure
values according to our standards no matter what, or not set them at
all.  This has really always been the case, but was harder to see at
the time because of interaction with other bugs.

Fixes #15927

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15996)
