tree-wide: remove unnecessary gcc >= 7 version check

Our baseline is gcc 8.4.

src/resolve/generate-dns_type-gperf.py

@@ -10,9 +10,7 @@ name, prefix, input = sys.argv[1:]
 
 print("""\
 %{
-#if __GNUC__ >= 7
 _Pragma("GCC diagnostic ignored \\"-Wimplicit-fallthrough\\"")
-#endif
 %}""")
 print("""\
 struct {}_name {{ const char* name; int id; }};

src/resolve/resolved-gperf.gperf

@@ -1,8 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 %{
-#if __GNUC__ >= 7
 _Pragma("GCC diagnostic ignored \"-Wimplicit-fallthrough\"")
-#endif
 #include <stddef.h>
 #include "conf-parser.h"
 #include "resolved-conf.h"