Bug 873569 - Follow-up: don't delete assets/omni.ja during unpack. r=me

The issue is that in INNER_UNMAKE_PACKAGE I was deleting
assets/omni.ja, which is needed in INNER_MAKE_PACKAGE.  This breaks
language repacks.  This fix should at least allow repacks to proceed
further.

toolkit/mozapps/installer/packager.mk

@@ -388,7 +388,6 @@ INNER_UNMAKE_PACKAGE	= \
     mv lib/$(ABI_DIR)/libmozglue.so . && \
     mv lib/$(ABI_DIR)/*plugin-container* $(MOZ_CHILD_PROCESS_NAME) && \
     mv $(ASSET_SO_LIBRARIES) . && \
-    rm -rf assets && \
     rm -rf lib/$(ABI_DIR) )
 endif