Remove dead code in toast_fetch_datum_slice

Remove dead code in toast_fetch_datum_slice

In toast_fetch_datum_slice(), we Assert() that what is passed in isn't
compressed, but we then later had a check to see what the length of if
what was passed in is compressed.  That later check is rather confusing
since toast_fetch_datum_slice() is only ever called with non-compressed
datums and the Assert() earlier makes it clear that one shouldn't be
passing in compressed datums.

Add a comment to make it clear that toast_fetch_datum_slice() is just
for non-compressed datums, and remove the dead code.