Remove content-type dependency

[samhh]

Our size-limit action is broken, but I see this locally:

$ git switch master
$ yarn install
$ yarn run build
$ yarn run size
 dist/unsplash-js.cjs.production.min.js
  Size limit: 5 KB
  Size:       4.21 KB with all dependencies, minified and gzipped

 dist/unsplash-js.esm.js
  Size limit: 5 KB
  Size:       4.22 KB with all dependencies, minified and gzipped

$ git switch content-type
$ yarn install
$ yarn run build
$ yarn run size
 dist/unsplash-js.cjs.production.min.js
  Size limit: 5 KB
  Size:       3.51 KB with all dependencies, minified and gzipped

 dist/unsplash-js.esm.js
  Size limit: 5 KB
  Size:       3.54 KB with all dependencies, minified and gzipped