Some events are not released in test_mem_host_flags

[aharon-abramson]

In test_mem_host_flags, some events are not released. In fact, they are not used, so they can be removed altogether.

[bashbaug]

This is an improvement, and I'm fine checking this in as-is, but it looks like there are still some unreleased events. Using the OpenCL Intercept Layer and LeakChecking, here is the report before this change:

No cl_context leaks detected.
No cl_command_queue leaks detected.
No cl_mem leaks detected.
Possible leak of type cl_event!
    Number of Allocations: 672
    Number of Retains:     0
    Number of Releases:    336

And after:

No cl_context leaks detected.
No cl_command_queue leaks detected.
No cl_mem leaks detected.
Possible leak of type cl_event!
    Number of Allocations: 432
    Number of Retains:     0
    Number of Releases:    336

Let me know if you 'd like help tracking down some of the remaining leaks.

[bashbaug]

Merging as discussed in the October 22nd teleconference. We will address the remaining leaks in a subsequent PR.