Remove always-empty variable

This has been empty since Ibdfe1cbf34d97c43f4857c2c5d7604fa72d5b5a4.

Test: m
Change-Id: I7ac973f82dc07fb74e0aae80c10a68d3ae6e1d72

Android.mk

@@ -32,10 +32,6 @@ ifneq ($(ANDROID_BUILD_EMBEDDED),true)
 # ============================================================
 include $(CLEAR_VARS)
 
-# This is used by ide.mk as the list of source files that are
-# always included.
-INTERNAL_SDK_SOURCE_DIRS := $(addprefix $(LOCAL_PATH)/,$(dirs_to_document))
-
 # sdk.atree needs to copy the whole dir: $(OUT_DOCS)/offline-sdk to the final zip.
 # So keep offline-sdk-timestamp target here, and unzip offline-sdk-docs.zip to
 # $(OUT_DOCS)/offline-sdk.