GUACAMOLE-249: Do not include CB_RESPONSE_OK flag in Format List PDU.

The CLIPRDR specification requires that the msgFlags field for the
Format List PDU be set to 0x0000. The function within FreeRDP overrides
this value to 0x0000, but it is still incorrect to attempt to set it.

src/protocols/rdp/clipboard.c

@@ -59,8 +59,7 @@ static UINT guac_rdp_cliprdr_send_format_list(CliprdrClientContext* cliprdr) {
             { .formatId = CF_TEXT },
             { .formatId = CF_UNICODETEXT }
         },
-        .numFormats = 2,
-        .msgFlags = CB_RESPONSE_OK
+        .numFormats = 2
     };
 
     return cliprdr->ClientFormatList(cliprdr, &format_list);