Remove unused EVP_PKEY_ASN1_METHODs and flags.

The only alias, EVP_PKEY_RSA2, is handled programmatically. ASN1_PKEY_ALIAS and
ASN1_PKEY_DYNAMIC are then unused and unexported and can be removed.

Change-Id: I990650636bac3b802c8b439257c67ce7a3f8bc70
Reviewed-on: https://boringssl-review.googlesource.com/2124
Reviewed-by: Adam Langley <agl@google.com>