Remove vendor & product in secondary payload

We used to carry the full OTA for vendor and product partitions when
generating the secondary payload. Since they are not actually used, we
can remove them to reduce the size of factory OTA packages. For example,
the size for a sargo package reduces from 454M to 283M after the change.

Bug: 140771390
Test: generate and apply a second payload on Pixel
Change-Id: I091dc07db8c345b182d9f5dd8d48f5c209245641