Remove experimental pass manager for fuzzer builds.

Sanitizer coverage is currently broken with the experimental pass
manager. See b/133876586 for more information. The patch is currently
being worked on upstream (https://reviews.llvm.org/D62888), but is not
ready yet. Hence, we disable it here (similar to the LTO bug).

Bug: 133876586
Test: Build anything with SANITIZE_TARGET='fuzzer', verify that it has
'sancov' symbols.

Change-Id: I74d02e52ccbe2a3d96b6b8e30230197b554e60a0