linux-user: remove useless variable

linux-user: remove useless variable

filename is only used to open the file if AT_EXECFD is not provided.
But exec_path already contains the path of the file to open.
Remove filename as it is only used in main.c whereas exec_path is
also used in syscall.c.

Fixes: d088d664f201 ("linux-user: identify running binary in /proc/self/exe")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190714134028.315-1-laurent@vivier.eu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>