gdb: remove add_alias_cmd overload that accepts a string

Same idea as previous patch, but for add_alias_cmd.  Remove the overload
that accepts the target command as a string (the target command name),
leaving only the one that takes the cmd_list_element.

gdb/ChangeLog:

	* command.h (add_alias_cmd): Accept target as
	cmd_list_element.  Update callers.

Change-Id: I546311f411e9e7da9302322d6ffad4e6c56df266

gdb/cli/cli-cmds.c

@@ -1908,8 +1908,7 @@ alias_command (const char *args, int from_tty)
 
       /* add_cmd requires *we* allocate space for name, hence the xstrdup.  */
       alias_cmd = add_alias_cmd (xstrdup (alias_argv[alias_argc - 1]),
-				 command_argv[command_argc - 1],
-				 class_alias, a_opts.abbrev_flag,
+				 target_cmd, class_alias, a_opts.abbrev_flag,
 				 c_command->subcommands);
     }