target: Remove error messages as no .get_gdb_fileio_info

target: Remove error messages as no .get_gdb_fileio_info

If target does not support semi-hosting function, it has
no need to provide .get_gdb_fileio_info callback. OpenOCD
will use default function target_get_gdb_fileio_info_default.
The default function just return ERROR_FAIL and gdb_server
will treat every halted condition as normal halted and
return "Txx" to gdb.

Change-Id: I9ddb2be3a1145eae2ef5b712bdea89eb2e0fbc20
Signed-off-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-on: http://openocd.zylin.com/1586
Tested-by: jenkins
Reviewed-by: Nemui Trinomius <nemuisan_kawausogasuki@live.jp>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>