Remove unused exceptions

Removed the exception classes in the following because not referenced
anywhere.

InvalidSourceVolume
InvalidRequest
PersistentVolumeFileNotFound
InvalidSnapshotMetadata
InvalidSnapshotMetadataSize
DiskNotFound
MigrationNotFoundByStatus
ClassNotFound
NotAllowed
KeyPairExists
MigrationError
VolumeSizeExceedsQuota
Duplicate3PARHost
BackupVolumeInvalidType

Change-Id: I7165f970752587885430f9d5c813b82b3f06ad70