Fixed `grpc_unsecure` CMake build failures due to non-standard openssl location.

`grpc_unsecure` doesn't use openssl, but one of its source file accidently
includes `openssl/rand.h`. Since `CMakeLists.txt` doesn't specify that
`grpc_unsecure` requires openssl, the build would fail. This can be fixed
by simply removing this header inclusion.

Fixed #11600.