Remove `pack_relocations: false`.

This was added to be able to test on O, but the proper change for O
compatibility would be to say `sdk_version: "26"` and let the build
system adjust everything that needs changing.

Remove these so we can remove `pack_relocations` entirely.

Bug: http://b/147452927
Test: treehugger
Change-Id: I4096209180c67128499e0850f2f21762e520b99a