logger_entry and logger_entry_v2 were used for the kernel logger,
which we have long since deprecated. logger_entry_v3 is the same as
logger_entry_v4 without a uid field, so it is trivially removable,
especially since we're now always providing uids in log messages.
liblog and logd already get updated in sync with each other, so we
have no reason for backwards compatibility with their format.
Test: build, unit tests
Change-Id: I27c90609f28c8d826e5614fdb3fe59bde22b5042
441054aa1e