Simplify return handling for tied handle OPEN/PRINTF/READ/WRITE.

Simplify return handling for tied handle OPEN/PRINTF/READ/WRITE.

Their code used to explicitly move the return value from its current position
on the stack, to the position that was top of top of the stack just before the
call was made. However, the POPBLOCK() in pp_leavesub will restore the stack
pointer, and passing G_SCALAR to call_method() will force exactly one return
value, so all of this is needless defensiveness.