linux-user: fix "apt-get update" on linux-user hppa

apt-get was hanging on linux-user hppa.

strace has shown the netlink data stream was not correctly byte swapped.

It appears the fd translator function is unregistered just after it
has been registered, so the translator function is not called.

This patch removes the fd_trans_unregister() after the do_socket()
in the TARGET_NR_socket case.

This fd_trans_unregister() was added by commit
    e36800c9 linux-user: add signalfd/signalfd4 syscalls
when do_socket() was not registering any fd translator.
And as now it is, we must remove this fd_trans_unregister() to keep them.

Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Message-Id: <20170126080449.28255-3-laurent@vivier.eu>
Signed-off-by: Richard Henderson <rth@twiddle.net>