Unexport ssl_get_new_session and ssl_update_cache.

Chromium's session cache has since been rewritten and no longer needs to
muck with those functions in tests.

Change-Id: I2defad81513210dca5e105757e04cbb677583251
Reviewed-on: https://boringssl-review.googlesource.com/4788
Reviewed-by: Adam Langley <agl@google.com>