Fix bad write in pp_trans

y/// has not been extending the stack for lexical $_, even since lex-
ical $_ was added.  $lexical =~ y/// has not been extending the stack
since v5.21.5-339-g05a502d.
pp.c:pp_trans: Remove targ variable

One spot was doing sv = GETTARG, which is equivalent to
sv = targ = ....  The value of targ was unused.  Elsewhere, a new mor-
tal was assigned to targ and then immediately pushed on to the stack.

Avoiding the redundant variable altogether results in smaller
machine code.
pp.c: pp_schop does not need PUTBACK

as it does no stack manipulation.
pp.c:pp_undef: Less stack fiddling

Only when called with no arguments does it need to manipulate the
stack pointer.