Remove incorrect Asserts in buffile.c

Remove incorrect Asserts in buffile.c

Both BufFileSize() and BufFileAppend() contained Asserts to ensure the
given BufFile(s) had a valid fileset.  A valid fileset isn't required in
either of these functions, so remove the Asserts and adjust the
comments accordingly.

This was noticed while work was being done on a new patch to call
BufFileSize() on a BufFile without a valid fileset.  It seems there's
currently no code in the tree which could trigger these Asserts, so no
need to backpatch this, for now.

Reviewed-by: Peter Geoghegan, Matthias van de Meent, Tom Lane
Discussion: https://postgr.es/m/CAApHDvofgZT0VzydhyGH5MMb-XZzNDqqAbzf1eBZV5HDm3%2BosQ%40mail.gmail.com