trusty: clear fuzzer's extra counters directly

The FuzzerDefs.h APIs are internal to the fuzzer and aren't available
when the fuzzer is built with a custom private libc++, so remove the
ExtraCountersBegin/ExtraCountersEnd assertions and inline the array
clearing.

Bug: 175635923
Bug: 303175229
Bug: 315079422
Test: m libtrusty_fuzz_utils trusty_gatekeeper_fuzzer
Change-Id: I1ca9d9867026ff6f8e494ac6026fb1314caab7d1