openssl: remove CONST_ASN1_BIT_STRING.

[davidben]

Just making the pointer as const works for the pre-1.1.0 path too.

[mention-bot]

@davidben, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bagder, @dmitrykos and @yangtse to be potential reviewers.

[coveralls]

Coverage decreased (-0.02%) to 75.09% when pulling e023ebf on davidben:const-asn1-bit-string into f87e77b on curl:master.

[jay]

In 1.0.2 X509_get0_signature takes non-const ASN1_BIT_STRING but the block in which it is called really only works for OpenSSL 1.1.0.

[davidben]

Right, but that's already the case due to &palg being always const. That block only runs for 1.1.0 anyway because of the HAVE_X509_GET0_EXTENSIONS check.

[bagder]

I'm all for this as it also removes complexity.

[bagder]

Thanks!