Revert "Replacing @Nullable with @RecentlyNullable for apis annotated since API 28"

This reverts commit 1b51142e now that nullability of non-android APIs will remain as @Nullable/@NonNull rather than @RecentlyNullable/@RecentlyNonNull

Bug: 141748642
Test: m api-stubs-docs &&
      cd out/target/common && mv docs docs-new && unzip docs-new/api-stubs-docs-stubs.srcjar -d docs-new/api-stubs-docs-stubs.srcjar_unzipped && cd - &&
      cd frameworks/base && git checkout HEAD^ && cd - && la m api-stubs-docs &&
      cd out/target/common && mv docs docs-old && unzip docs-old/api-stubs-docs-stubs.srcjar -d docs-old/api-stubs-docs-stubs.srcjar_unzipped &&
      diff -q -x "*.html" -r docs-old/ docs-new # And notice that all of the changes are in android/ or in dalvik/

Change-Id: Iecad0acdd910de4a1270e3ccfe34fe378d22a113