ENCODER: Don't pass libctx to OSSL_ENCODER_CTX_new_by_EVP_PKEY()

ENCODER: Don't pass libctx to OSSL_ENCODER_CTX_new_by_EVP_PKEY()

The passed 'pkey' already contains a library context, and the encoder
implementations should be found within the same context, so passing an
explicit library context seems unnecessary, and potentially dangerous.

It should be noted that it's possible to pass an EVP_PKEY with a
legacy internal key.  The condition there is that it doesn't have a
library context assigned to it, so the NULL library context is used
automatically, thus requiring that appropriate encoders are available
through that context.

Fixes #13544

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13545)