Plug source_command leak

Plug source_command leak

The heap-allocated 'old_source_verbose' local was accidentally left
behind by commit 2ec845e75876 ("More uses of scoped_restore").

Valgrind caught it, like:

 ==20123== 8 bytes in 1 blocks are definitely lost in loss record 4,609 of 13,785
 ==20123==    at 0x4C2A988: calloc (vg_replace_malloc.c:711)
 ==20123==    by 0x60A2F8: xcalloc (common-utils.c:84)
 ==20123==    by 0x4CDBE5: build_command_line(command_control_type, char const*) (cli-script.c:159)
 ==20123==    by 0x4CDC32: get_command_line(command_control_type, char const*) (cli-script.c:172)
 ==20123==    by 0x5230F1: python_command(char*, int) (python.c:421)
 ==20123==    by 0x4C61AD: do_cfunc(cmd_list_element*, char*, int) (cli-decode.c:106)
 ==20123==    by 0x4C911F: cmd_func(cmd_list_element*, char*, int) (cli-decode.c:1902)
 ==20123==    by 0x7CA79E: execute_command(char*, int) (top.c:650)
 ==20123==    by 0x695A0C: command_handler(char*) (event-top.c:590)
 ==20123==    by 0x7CA33F: read_command_file(_IO_FILE*) (top.c:461)
 ==20123==    by 0x4D0C3A: script_from_file(_IO_FILE*, char const*) (cli-script.c:1584)
 ==20123==    by 0x4C2727: source_script_from_stream(_IO_FILE*, char const*, char const*) (cli-cmds.c:589)

gdb/ChangeLog:
2017-08-17  Pedro Alves  <palves@redhat.com>

	* cli/cli-cmds.c (source_command): Delete 'old_source_verbose'
	local.