Trim unused cipher indices and definitions.

Also remove SSL_eNULL ciphers. They were broken anyway in the initial import
because of a lost 'else', but just remove them altogether.

Change-Id: Ie71cf1b45f8fc6883e209801443eddf7f2d058ba
Reviewed-on: https://boringssl-review.googlesource.com/1518
Reviewed-by: Adam Langley <agl@google.com>