Remove the OSSL_DEBUG flag and OpenSSL.mem_check_start which is only
compiled when the flag is given. They are meant purely for development
of Ruby/OpenSSL.

OpenSSL.mem_check_start helped us find memory leak bugs in past, but
it is no longer working with the recent OpenSSL versions. Let's just
remove it now.

https://github.com/ruby/openssl/commit/8c7a6a17e2