Fix version numbers for some GnuTLS features

Problem reported by Glenn Morris (Bug#27708#58).
* src/gnutls.c (HAVE_GNUTLS_X509_SYSTEM_TRUST):
New macro.  Use it instead of low-level version number checks.
(HAVE_GNUTLS_AEAD): Move here from gnutls.h, and rename from
HAVE_GNUTLS3_AEAD.  All uses changed.  Indent preprocessor lines.
* src/gnutls.h (HAVE_GNUTLS3_CIPHER, HAVE_GNUTLS3_DIGEST)
(HAVE_GNUTLS3_HMAC): Remove, since these were available
before GnuTLS 3.0.0 and the code checks them only if HAVE_GNUTLS3
is defined.  Remove all uses; this simplifies the code a bit.

src/gnutls.h

@@ -28,22 +28,6 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 # include <gnutls/crypto.h>
 #endif
 
-#if 0x030400 <= GNUTLS_VERSION_NUMBER
-# define HAVE_GNUTLS3_CIPHER
-# define HAVE_GNUTLS3_DIGEST
-# define HAVE_GNUTLS3_HMAC
-#endif
-
-/* Although AEAD support started in GnuTLS 3.4.0 and works in 3.5.14,
-   it was broken through at least GnuTLS 3.4.10; see:
-   https://lists.gnu.org/archive/html/emacs-devel/2017-07/msg00992.html
-   The relevant fix seems to have been made in GnuTLS 3.5.1; see:
-   https://gitlab.com/gnutls/gnutls/commit/568935848dd6b82b9315d8b6c529d00e2605e03d
-   So use 3.5.1 for now.  */
-#if 0x030501 <= GNUTLS_VERSION_NUMBER
-# define HAVE_GNUTLS3_AEAD
-#endif
-
 #include "lisp.h"
 
 /* This limits the attempts to handshake per process (connection).  It