[ci] Remove pointless `$MAKE_TARGETS` -

1. there was an odd error 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`.
Merge pull request #186 from errordeveloper/remove_make_targets_env_var_for_travis

[ci] Remove pointless `$MAKE_TARGETS`