Remove dead gradle code.
Remove dead gradle code.

We never called generateSourceProps task. It was supposed to be
generating source.properties files, but we never had them and nothing
in sdk manager is broken, so let's remove these.

Test: ./gradlew createArchives
Bug: 38177084
Change-Id: I27c8f1e6607641fdfb669e5217bbca542a1110c2
(cherry picked from commit 7d4e88121af312d921f57551ed623b26e1c8d556)

AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,aosp-mirror/platform_frameworks_support