releasetools: ota_from_target_files: fix build for A-Only dynamic devices not building vendor

The function `HasVendorParition()` was moved inside `HasTrebleEnabled()` in 9276cf02
It also has a different implementation now

`HasPartition()` which was used by it earlier is still present, using the same implementation, so use that directly

Fixes the following error

Traceback (most recent call last):
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/internal/stdlib/runpy.py", line 174, in _run_module_as_main
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/internal/stdlib/runpy.py", line 72, in _run_code
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/__main__.py", line 12, in <module>
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/internal/stdlib/runpy.py", line 174, in _run_module_as_main
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/internal/stdlib/runpy.py", line 72, in _run_code
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/ota_from_target_files.py", line 2300, in <module>
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/ota_from_target_files.py", line 2279, in main
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/ota_from_target_files.py", line 2019, in GenerateNonAbOtaPackage
  File "/home/akhil/eleven/out/host/linux-x86/bin/ota_from_target_files/ota_from_target_files.py", line 855, in WriteFullOTAPackage
NameError: global name 'HasVendorPartition' is not defined

Change-Id: I9bd4ebc9d6b103cfd572bfd5eacd3f7a5b1a661c
Signed-off-by: Akhil Narang <me@akhilnarang.dev>