Remove git submodules to simplyfy the workflow

[ctwhome]

Terria has been moved to version 8, we are using version 7 and at this point it is not possible to migrate to the new version.

Since we never updated the submodule from terriajs, I suggest making terriajs part of this repository and removing the git submodules configuration, which only makes the development experience a bit harder.

Resulting in

One repository for TerriaMap and TerriaJS