include/sys: Remove unused SYS_nnetsocket from syscall.h
Because SYS_nnetsocket is not implemented so far, it should be
removed so that SYS_prctl can work correctly.
Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>
Approved-by: GregoryN <gnutt@nuttx.org>