core: Remove CanUpdateFieldGeneric

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

This patch removes the following:
1. The CanUpdateFieldGenericQuery class
2. The CanUpdateFieldGeneric constant representing
   it in VdcQueryType
3. The CanUpdateFieldGenericParameters class which
   was only used by the removed query class.

Change-Id: I644d8b5c6854de5c14b3a93fd37c2e77666a890e
Signed-off-by: Allon Mureinik <amureini@redhat.com>