releasetools: Default journal size to 0 for system images.

This CL defaults the journal size to 0 for system images (i.e. system,
vendor, system_other, oem, product). We used to do this by explicitly
defining BOARD_{SYSTEM,VENDOR,OEM,PRODUCT}IMAGE_JOURNAL_SIZE to 0 in
device-specific BoardConfig. Targets can still specify a non-zero
journal size as needed.

With this CL, marlin/sailfish gets 32MB / 16MB free space back for
system and vendor respectively.

Bug: 75975085
Test: `m dist`. Check the journal size in the generated images.
Change-Id: Ib3185d07c49be2b4f0fac5fe17ec1a82093ba0c0