toke.c: PL_in_eval purge

toke.c: PL_in_eval purge

Many uses of PL_in_eval in toke.c are redundant.

PL_in_eval indicates not that we are parsing a string eval, but that
we are being called from an eval, whether stringy on not.  Even if
PL_in_eval were only for string eval, it would still not indicate that
we are parsing a string eval, because of eval 'require'.

This commit removes redundant uses of it (making things theoretically
slightly faster).