Check FIPS_mode() rather than explicitly calling BORINGSSL_self_test.

After this CL topic, BORINGSSL_self_test runs automatically when
libcrypto is loaded. It is no longer possible or necessary to
start it manually. Instead, we check that we're running in FIPS_mode(),
which guarantees that the self test has already run.

Bug: 137267623
Bug: 139918106
Test: Treehugger
Change-Id: Ic3564218de4f01c37c51c139b1ad5a435a1fd6cd