DigestTest: remove performance comparison

Do not assert that OpenSSL digest implementations are faster. A CTS test is
not the right place to do it, plus the measurements in the context of CTS
runs are inaccurate and that's causing the test to be flaky ( b/29532930 ).

As an aside, performance is not going to change by N and for O we are
planning to do this measurements in benchmarks ( b/29816930 ).

Bug: 29532930
Bug: 29816930
Change-Id: Ifc4c912ed97f754d070bd12ff88359fc3cc319bf