Remove BN_LONG macro.

It's never used, upstream or downstream. The 64-bit value is wrong anyway for
LLP64 platforms.

Change-Id: I56afc51f4c17ed3f1c30959b574034f181b5b0c7
Reviewed-on: https://boringssl-review.googlesource.com/2123
Reviewed-by: Adam Langley <agl@google.com>