syscall.master: Remove stray 4.2

Back in 4.3BSD, the system call table wasn't generated, and there was an
entry:

"4.2 sigreturn",        /* 139 = old 4.2 sigreturn */

This got converted to
139 OBSOL 0 4.2 sigreturn
in 4.3 RENO. Since it was obsolete, nothing bad happened. In fact,
there was code in makeyscalls.sh to cope:

{       comment = $4
        for (i = 5; i <= NF; i++)
                comment = comment " " $i
        if (NF < 5)
                $5 = $4
}

so the generated comment in syscalls.c was almost correct:

"obs_4.2",                      /* 139 = obsolete 4.2 sigreturn */

a bug that we have to this very day, despite makesyscalls.sh being
rewritten in lua.

However, this historical wart is the only place in our current
syscalls.master file where we have an extra field for the 'not
generated' class of system calls. Remove the historical wart so that the
re-write of makesyscalls.lua can be simpler (so, I hope, qemu's bsd-user
can large swathes of code automatically generated too). This should help
make things more understandable (changes to simplify makesyscalls.lue
aren't quite debugged, so have to wait for another day).

There's 3 different obsolete sigreturns (but only 1 that was ever in
FreeBSD 2.x and newer).

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