[ci] Remove pointless `$MAKE_TARGETS`

1. there was [an odd error](https://travis-ci.org/contiki-os/contiki/jobs/5741797#L95) when this environment variable wasn't set (`[: =: unary operator expected`)
2. this environment variable is not used anywhere else

Removing it makes the logic look much simpler - do make `cooja.jar` if build type is any other then `compile`.