E2S1: Use [modify_unit] instead of trivial store/unstore sequences

Internally, [modify_unit] is implemented using store/unstore and a load
of other cruft. However, since the relevant portions of E2S1 are
one-time-only code, that doesn't really matter and we can save up some
WML nodes this way.

Incidentally, this solves an elynia_store variable leaking through the
first few turns of E2S1 until replaced and cleared by the village
entry event.