gnutls: remove call to deprecated gnutls_compression_get_name

It has been deprecated by GnuTLS since a year ago and now causes build
warnings.

Ref: https://gitlab.com/gnutls/gnutls/commit/b0041897d2846737f5fb0f
Docs: https://www.gnutls.org/manual/html_node/Compatibility-API.html

Closes #3636