tools:discoverer: Remove short name for use-cache

There is no need to add short name for 'use-cache'.
The 'async' option already has 'a' short name.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/716>

tools/gst-discoverer.c

@@ -637,7 +637,7 @@ main (int argc, char **argv)
   GOptionEntry options[] = {
     {"async", 'a', 0, G_OPTION_ARG_NONE, &async,
         "Run asynchronously", NULL},
-    {"use-cache", 'a', 0, G_OPTION_ARG_NONE, &use_cache,
+    {"use-cache", 0, 0, G_OPTION_ARG_NONE, &use_cache,
         "Use GstDiscovererInfo from our cache.", NULL},
     {"print-cache-dir", 0, 0, G_OPTION_ARG_NONE, &print_cache_dir,
         "Print the directory of the discoverer cache.", NULL},