ossl_seed: remove leftover RAND_screen check

Before commit 2dded8fe (dec 2010) there was logic that used
RAND_screen() at times and now I remove the leftover #ifdef check for
it.

The seeding code that uses Curl_FormBoundary() in ossl_seed() is dubious
to keep since it hardly increases randomness but I fear I'll break
something if I remove it now...