gnu: gnutls: Update to 3.6.15 and remove replacement.
* gnu/packages/tls.scm (gnutls): Update to 3.6.15.
[source]: Add "gnutls-cross.patch".
[replacement]: Remove.
[native-inputs]: Add GUILE-3.0 when (%current-target-system) is true.
(gnutls/fixed): Remove.
* gnu/packages/package-management.scm (guix)[propagated-inputs]: Remove
reference to 'gnutls/fixed'.
* gnu/packages/tls.scm (gnutls/dane): Inherit from gnutls.
* gnu/packages/vpn.scm (openconnect)[propagated-inputs]: Remove
gnutls/fixed, add gnutls.
Co-authored-by: Efraim Flashner <efraim@flashner.co.il>