Remove SSL_CTX_get_extra_chain_certs_only.

This is in preparation for folding away certificate slots. extra_certs
and the slot-specific certificate chain will be the same.
SSL_CTX_get_extra_chain_certs already falls back to the slot-specific
chain if missing. SSL_CTX_get_extra_chain_certs_only is similar but
never falls back. This isn't very useful and is confusing with them
merged, so remove it.

BUG=486295

Change-Id: Ic708105bcf453dfe4e1969353d7eb7547ed2981b
Reviewed-on: https://boringssl-review.googlesource.com/5350
Reviewed-by: Adam Langley <agl@google.com>
2 files changed