test/drbgtest.c: Remove error check for large generate requests

test/drbgtest.c: Remove error check for large generate requests

The behaviour of RAND_DRBG_generate() has changed. Previously, it
would fail for requests larger than max_request, now it automatically
splits large input into chunks (which was previously done only
by RAND_DRBG_bytes() before calling RAND_DRBG_generate()).

So this test has not only become obsolete, the fact that it succeeded
unexpectedly also caused a buffer overflow that terminated the test.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11195)