pp_formline: don't do get mg on PL_formtarget

pp_formline: don't do get mg on PL_formtarget

Two of the three branches that upgrade PL_formtarget to utf8 (FF_LITERAL,
FF_ITEM) do get magic while doing so, while the third (FF_LINEGLOB)
doesn't. I think the first two were just co-incidental
(they started out as simple sv_utf8_upgrade() calls added by
1bd51a4ce2ce8, and probably no consideration was given as to whether to
use the _nomg variant instead).

Make the first two no longer call magic, to be consistent with
FF_LINEGLOB.