GTK: prefer using curl's intrinsic defaults for CURLOPT_CA*

On distributions that ship only a ca-bundle in `/etc/ssl`, the
configuration set at initialization will force curl to look for
certificates that are not present in this directory.

Removing the setting instead makes curl use its internal default.

The framebuffer frontend will not set this option, and the only
other frontend that sets it is BeOS.

This issue can be observed (likely among other) on the NixOS Linux
distribution.

frontends/gtk/gui.c

@@ -200,15 +200,11 @@ static nserror set_defaults(struct nsoption_s *defaults)
 		nsoption_setnull_charp(downloads_directory, strdup(fname));
 	}
 
-	/* default path to certificates */
-	nsoption_setnull_charp(ca_path, strdup("/etc/ssl/certs"));
-
 	if ((nsoption_charp(cookie_file) == NULL) ||
 	    (nsoption_charp(cookie_jar) == NULL) ||
 	    (nsoption_charp(url_file) == NULL) ||
 	    (nsoption_charp(hotlist_path) == NULL) ||
-	    (nsoption_charp(downloads_directory) == NULL) ||
-	    (nsoption_charp(ca_path) == NULL)) {
+	    (nsoption_charp(downloads_directory) == NULL)) {
 		NSLOG(netsurf, INFO,
 		      "Failed initialising default resource paths");
 		return NSERROR_BAD_PARAMETER;