core: Remove unused IsStoragePoolWithSameNameExist

Removed the unused IsStoragePoolWithSameNameExistQuery.
This patch removes the following:
1. The unused IsStoragePoolWithSameNameExistQuery class.
2. The IsStoragePoolWithSameNameExistParameters class only used by the
   aforementioned query class.
3. The IsStoragePoolWithSameNameExistQuery constant in VdcQueryType
   corresponding to the removed class.

Change-Id: I08bba52c8795bba0535d4651c0b6532dd4d523c9
Signed-off-by: Allon Mureinik <amureini@redhat.com>