Revert "Make rehash(0) work with ubsan's unsigned-integer-overflow."

This caused some libc++ test failures. Will replace this with just
avoiding ubsan instrumentation on this function.

This reverts commit 4e689750214209b1de74bbfd512eb28099f22caa.

Test: make checkbuild
Test: ./run_tests.py
Bug: http://b/69772346
1 file changed