bootctl: don't start "bootctl list" output with a title line

Let's not output a title line given that we don't output multiple
different things here anyway, and the "bootctl list" command is about
listing boot entries anyway and it's documented that way.

Having titles for sections if we have mutliple sections of output
definitely makes sense, but if there's only one kind of information we
list it's redundant clutter.

src/shared/bootspec.c

@@ -1399,6 +1399,8 @@ int show_boot_entry(
 int show_boot_entries(const BootConfig *config, JsonFormatFlags json_format) {
         int r;
 
+        assert(config);
+
         if (!FLAGS_SET(json_format, JSON_FORMAT_OFF)) {
                 for (size_t i = 0; i < config->n_entries; i++) {
                         _cleanup_free_ char *opts = NULL;
@@ -1443,8 +1445,6 @@ int show_boot_entries(const BootConfig *config, JsonFormatFlags json_format) {
                 }
 
         } else {
-                printf("Boot Loader Entries:\n");
-
                 for (size_t n = 0; n < config->n_entries; n++) {
                         r = show_boot_entry(
                                         config->entries + n,