Remove overzealous _bt_split() assertions.

Remove overzealous _bt_split() assertions.

_bt_split() is passed NULL as its insertion scankey for internal page
splits.  Two recently added Assert() statements failed to consider this,
leading to a crash with pg_upgrade'd BREE_VERSION < 4 indexes.  Remove
the assertions.

The assertions in question were added by commit 0d861bbb, which added
nbtree deduplication.  It would be possible to fix the assertions
directly instead, but they weren't adding much anyway.