Invoke delta_generator directly

Currently, ota_from_target_files(a python script) calls
brillo_update_payload(a bash script), which then calls delta_generator(a
C++ binary) to do the actual diffing. Having brillo_update_payload in
the middle does not offer any additional flexibility, but it makes
maintaince more difficult. Bash code is less readable and harder to
debug. Further more, everytime we want to add a new flag to
delta_generator, we have to add the flag in three places:
ota_from_target_files, brillo_update_payload, delta_generator.

Historically, brillo_update_payload was there because we inherited from
ChromeOS side. This bash scripts extracts target_files.zip and invoke
delta_generator to generate the actual OTA. Any customizations we want
on OTA must be implemented by modifying the input target_files.zip ,
Manipuating big zip files is slow and inefficient.

To make thing simpler/faster, remove the intermdiary call to
brill_update_payload. ota_from_target_files will now extract target
files and call delta_generator directly.

Test: th
Bug: 227848550
Change-Id: I44b296e43bf7921bcf48ef6a1e4021a12669739c