Remove UPDATE_SUBMODULES
Remove UPDATE_SUBMODULES

Implicit submodule updates causes much confusion among users. Explicit
is better than implicit. The documenation on compiling will now
recommend cloning with --recursive. Developers will need to manually
manage submodules.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue