remove ZF2 submodule

Zend Framework is being installed both by Composer and git submodule.
It makes confusion with two ZF's (/vendor/ZF2 and /vendor/zendframework).

As Composer is recommended way to deploy skeleton, let's remove the submodule.

Even if developer does not run Composer to install skeleton,
he may want to install another libraries with it.
Those libaries (e.g. Doctrine) are dependant on zendframework/zendframework
package, so it will result in two ZF's anyways.