Add Google Maps JS only where needed

Currently the module loads the Google Maps JS on every page, even if on the page there is any map.

Steps to reproduce

1. create a new block type
2. add a geofield field on the new block type
3. set the styled google map visualization style
4. create a block of the newly created type
5. add the block only on the front page
6. open a different page and check the source code.

To fix

Move the two external JS libraries into style_google_map.libraries.yml:

styled-google-map:
  version: 1.0
  js:
    //maps.google.com/maps/api/js: { type: external }
    //cdn.rawgit.com/googlemaps/js-info-bubble/gh-pages/src/infobubble.js: { type: external }
    js/styled-google-map.js: {}
  css:
    css/styled-google-map.css : {}
  dependencies:
    - core/jquery
    - core/drupal
    - core/drupalSettings

Then remove the styled_google_map_page_attachments() function from the .module file.

The map still works and now the JS is loaded only if necessary.