Remove sv_prepsyscall, sv_sigsize and sv_sigtbl members of the struct

Remove sv_prepsyscall, sv_sigsize and sv_sigtbl members of the struct
sysent.

sv_prepsyscall is unused.

sv_sigsize and sv_sigtbl translate signal number from the FreeBSD
namespace into the ABI domain.  It is only utilized on i386 for iBCS2
binaries.  The issue with this approach is that signals for iBCS2 were
delivered with the FreeBSD signal frame layout, which does not follow
iBCS2.  The same note is true for any other potential user if
sv_sigtbl.  In other words, if ABI needs signal number translation, it
really needs custom sv_sendsig method instead.

Sponsored by:	The FreeBSD Foundation