releasetools: Drop the support for file-based OTA generation.

We have stopped shipping devices with file-based OTAs, and are not
actively maintaining the support. Devices using file-based OTAs
should be moved to block-based, if not A/B OTAs.

We will also need to clean up EdifyGenerator class, which will be
handled in follow-up CLs.

Bug: 35853185
Test: Generate full and incremental OTAs w/ and w/o the CL, and they
      give identical packages.
Test: Not specifying --block also generates block-base OTAs.
Change-Id: I3b0fc8ce5600e109f3251fe41f655534aaa298c7