Drop in-tree dependency checker (#637)

The module shipped its own bootstrap-time dependency checker that
inspected installed library versions and, on failure, redirected the
user to a `vspheredb/phperror/dependencies` page. Icinga Web already
surfaces module-load failures, so this scaffolding is redundant.

Mirrors the equivalent cleanup in icingaweb2-module-director (see
PR #3065). The `module.info` `Depends:` line is intentionally left
unchanged.

Author: lippserd