Remove InputRootAbsolutePath spec to header-abi-dumper

We used this when doing cache-only for header-abi-dumper to prevent
cross-branch cache hits since abi-dumper had the absolute path of the files
in its output. Since header-abi-dumper no longer outputs absolute paths,
we no longer need to prevent cross-branch cache-hits.

Bug: b/162045672
Change-Id: Iecc3ad9614cda6202cc5f8a9d538f806a79bdf3b