Revert "Display a warning when a module is already defined and proceed to define the new module.  This will allow builds to proceed when receiving an error such as build/core/base_rules.mk:130: *** device/samsung/d2-common/rootdir: MODULE.TARGET.ETC.init.target.rc already defined by device/htc/ville/rootdir.  Stop."

This reverts commit 534fd0690926494409d6272c6a3784f149d801a2.