Prevent `weight = "auto"` from getting exported to Markdown for per-file exports

[Kaushal Modi]

How to recreate the problem:

#+HUGO_BASE_DIR: /home/kmodi/sandbox/org/
#+HUGO_SECTION: ./
#+HUGO_WEIGHT: auto
#+HUGO_AUTO_SET_LASTMOD: t

* KV

** WhiteDB

   test

Do C-c C-e H h (per-file export).

I get:

+++
lastmod = 2017-09-28T09:44:06-04:00
weight = "auto"
draft = false
+++

## KV {#kv}


### WhiteDB {#whitedb}

test

That weight = "auto" should not be there.