kleaf: use rel_path everywhere.

rel_path is a function in build_utils.sh (used by both
build.sh and Bazel) to determine the relative path between
two directories.

This removes the Python hack. The hack was introduced when
build_utils.sh was not created.

Test: TH
Bug: 208889801
Change-Id: Ic31b74a9a93a4f9d798f9cc9878b51a05a830b3c
1 file changed