rgw: remove unnecessary OpsLogFile::flush_mutex

rgw: remove unnecessary OpsLogFile::flush_mutex

this mutex was only held by one function, OpsLogFile::flush(). this
private member function is only ever called from the background thread,
so doesn't need to be protected by a mutex

as a further cleanup, i renamed 'cond' and 'mutex' now that we don't
need to differentiate between different locks

Signed-off-by: Casey Bodley <cbodley@redhat.com>