The distinction was not well-enforced in the code. In fact, it wasn't
even possible to use the RSA_SIGN slot because ssl_set_pkey and
ssl_set_cert would always use the RSA_ENC slot.
A follow-up will fold away the mechanism altogether, but this is an easy
initial simplfication.
BUG=486295
Change-Id: I66b5bf3e6dc243dac7c75924c1c1983538e49060
Reviewed-on: https://boringssl-review.googlesource.com/5349
Reviewed-by: Adam Langley <agl@google.com>
bb20f52383