Remove ca_mutex_trylock and associated unit test

Remove ca_mutex_trylock and associated unit test

ca_mutex_trylock is not used anywhere. Implementation of this function
presents challenges for porting to additional platforms where an
easily-wrappable implementation might not be available, such as on
Windows.

Change-Id: I7727c5f9a6c97a53dda15007cdfd74043ab03a7b
Signed-off-by: David Antler <david.a.antler@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/5645
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jon@joncruz.org>