gnutls: Fix nettle discovery

gnutls: Fix nettle discovery

Commit e06fa7462ac258c removed support for libgcrypt leaving only
support for nettle which has been the default crypto library in
GnuTLS for a long time. There were however a few conditionals on
USE_GNUTLS_NETTLE which cause compilation errors in the metalink
code (as it used the gcrypt fallback instead as a result). See the
below autobuild for an example of the error:

  https://curl.se/dev/log.cgi?id=20210225123226-30704#prob1

This removes all uses of USE_GNUTLS_NETTLE and also removes the
gcrypt support from the metalink code while at it.

Closes #6656
Reviewed-by: Daniel Stenberg <daniel@haxx.se>