Remove unused/deprecated error classes

It seems a few of the error classes are no longer used:

- LVMError, deprecated in 8c5533a5 (before ganeti 1.2.2!)
- ConfdRequestError, deprecated in b0dcdc10
- SshKeyError, introduced in the initial open source commit but never used (⁈)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>