Remove Circle CI support

[hectcastro]

Removes the configuration file used by Circle CI.

I used the commit history for circle.yml to ensure that any other changes made within the context of Circle CI were accounted for:

• https://github.com/OpenTreeMap/otm-core/commit/c321a96a08ac921bd5d331c26855aef4a49694b3
• https://github.com/OpenTreeMap/otm-core/commit/28d5ce05e9138030b1cc52c281b53dc2ae0bc392
• https://github.com/OpenTreeMap/otm-core/commit/7dbf5f23374e970f059efe7e92fa4e87ab1b0a01
• https://github.com/OpenTreeMap/otm-core/commit/e7da9f606577b7e638b9d4d74b8281eaa1f041ea

In addition, I went into the Circle CI UI and stopped otm-core jobs from building.

Fixes https://github.com/OpenTreeMap/otm-cloud/issues/452

[coveralls]

Coverage remained the same at 83.796% when pulling 43c9c8b0ccbfdabc3bc7235bc9fe8fb1f04c8cbd on feature/hmc/remove-circle-ci into 0276cda348d7215d8df5b11ef9b00c47587eac51 on develop.