SAVEt_CLEARSV: simplify SvREADONLY_off() condition

SAVEt_CLEARSV: simplify SvREADONLY_off() condition

SAVEt_CLEARSV should always be called with SvPADMY() true, so don't test
for it, but assert it instead. Then change this:

    if (SvPADMY(sv) && !SvFAKE(sv))
         SvREADONLY_off(sv);

The SvPADMY() isn't needed, but we should test for RO-ness instead, to
avoid an unnecessary SvREADONLY_off().