Do not copy QRCode images when building blog-utils lib

[willdurand]

Fixes mozilla/addons#14311

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With this patch, we don't have the QR code images anymore:

$ ls -l dist/
total 3392
-rw-r--r--  1 williamdurand  staff   112B Oct  8 08:47 README.md
-rw-r--r--  1 williamdurand  staff   925K Oct  8 08:48 node.js
-rw-r--r--  1 williamdurand  staff   416B Oct  8 08:47 package.json
-rw-r--r--  1 williamdurand  staff   147K Oct  8 08:48 style.css
-rw-r--r--  1 williamdurand  staff   612K Oct  8 08:48 web.js

[codecov]

Codecov Report

Merging #10956 (5679452) into master (896a3b1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   mozilla/addons-frontend#10956   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         258      258           
  Lines        7540     7540           
  Branches     1367     1367           
=======================================
  Hits         7419     7419           
  Misses        113      113           
  Partials        8        8           

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