Remove unused positionType - Resolves #26

[Alek-S]

Minor PR to resolve #26 by removing positionType from src/pages/job-listings/index.js

More Details: Unused positionType causes compile and related routing error, since it doesn't have a typedef.

ex:

ℹ ｢wdm｣: Compiling...
GraphQL Error Unknown field `positionType` on type `JobListingsJson`

  file: /Users/aleksandr.shnayder/code/chicagojs.org/src/pages/job-listings/index.js

   1 |
   2 |   query JobListingsQuery {
   3 |     allJobListingsJson {
   4 |       edges {
   5 |         node {
   6 |           id
   7 |           neighborhood
   8 |           position
   9 |           datePosted
> 10 |           positionType
     |           ^
  11 |         }
  12 |       }
  13 |     }
  14 |   }
  15 |