releasetools: Remove some legacy paths for prop rewrites.

sign_target_files_apks.py only needs to take care of the current
release. The legacy paths of ODM/build.prop, VENDOR/odm/build.prop, and
BOOT/RAMDISK/default.prop no longer exist in the target_files.zip from
master.

The other two cases of ROOT/default.prop and
RECOVERY/RAMDISK/default.prop are still kept in the code, as they will
still exist (as symlink or conditionally).

Test: Run sign_target_files_apks.py against
      aosp_taimen-target_files.zip. Check the rewritten prop files.
Test: `python -m unittest test_sign_target_files_apks`
Change-Id: I5e70bc2ccc0f3dcf0eace0718c59a3b0f89a9ff4