Don't pass down nonnullable_vars while reducing outer joins.

Don't pass down nonnullable_vars while reducing outer joins.

We weren't actually using the passed-down list for anything, other
than computing the new value to be passed down further.  I (tgl)
probably had the idea that we'd need this data eventually; but
no use-case has emerged in a good long while, so let's just stop
expending useless cycles here.

Richard Guo

Discussion: https://postgr.es/m/CAMbWs48KLy9aBb=sZ5MoNmnqAcGHaW_JTGWLCgoE_uMW7S6C-A@mail.gmail.com