Remove extraneous code, particularly from earlier examples

[jashmenn]

Earlier days (take day-01, for example) have a lot of extra code that isn't necessary. This is just confusing for folks trying to work through the code.

For example, day-01 just loads an HTML file, but the instructions say to use yarn, there's a postcss.config.js etc. This is totally unnecessary for that day.

The task here is to go through each day and ensure we only have exactly the code we need.

[jashmenn]

Also, on day-01, why do we have both:

• https://github.com/fullstackreact/30-days-of-react/blob/master/day-01/post.md
  and
• https://github.com/fullstackreact/30-days-of-react/blob/master/day-01/doc/doc.mkd ?

it seems to me we only need one or the other.