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

cc/builder.go

@@ -220,7 +220,6 @@ var (
 			ExecStrategy: "${config.REAbiDumperExecStrategy}",
 			Platform: map[string]string{
 				remoteexec.PoolKey:      "${config.RECXXPool}",
-				"InputRootAbsolutePath": android.AbsSrcDirForExistingUseCases(),
 			},
 		}, []string{"cFlags", "exportDirs"}, nil)