core: Remove GetAvailableClustersByServerCpu

GetAvailableClustersByServerCpuQuery is not used anywhere in
the code, and hence should be removed.

This patch removes the following:
1. The GetAvailableClustersByServerCpuQuery class.
2. The GetAvailableClustersByServerCpu constant representing
   it in VdcQueryType.
3. The GetAvailableClustersByServerCpuParameters class only used by
   the aforementioned query.
4. Methods in CpuFlagsManagerHandler only used by the
   aforementioned query: GetAllServerCpusBelowCpu(String, Version)
   and GetAllServerCpusBelowCpu(String).

Change-Id: Id8bca65412e18c2c62ead41825b08cddd9506c11
Signed-off-by: Allon Mureinik <amureini@redhat.com>