Remove OPENSSL_NO_SHA256

Changes were made by running

find . -type f -name *.c | xargs unifdef -m -U OPENSSL_NO_SHA256
find . -type f -name *.h | xargs unifdef -m -U OPENSSL_NO_SHA256

Change-Id: Ia2fda4b1a3a4aad286ced86cf936358fc7a30b15
Reviewed-on: https://boringssl-review.googlesource.com/1378
Reviewed-by: Adam Langley <agl@google.com>