Fix rapid7#5244, Remove unnecessary check for mysql_drop_and_create_sys_exec

Fix rapid7#5244, MySQL is always return OK so it doesn't seem to be so
important to check res for DROP FUNCTION IF EXISTS sys_exe

Author: wchen-r7

lib/msf/core/exploit/mysql.rb

@@ -162,8 +162,9 @@ def mysql_upload_sys_udf(arch=:win32,target_path=nil)
   end
 
   def mysql_drop_and_create_sys_exec(soname)
-    res = mysql_query("DROP FUNCTION IF EXISTS sys_exec") # Already checked, actually
-    return false if res.nil?
+    # Just drop it. MySQL will always say "OK" anyway.
+    # See #5244
+    mysql_query("DROP FUNCTION IF EXISTS sys_exec")
 
     res = mysql_query("CREATE FUNCTION sys_exec RETURNS int SONAME '#{soname}'")
     return false if res.nil?