Island: Remove dependency from ILockableEncryptor -> IEncryptor

ILockableEncryptor adds additional constraints on when encrypt() or
decrypt() can be used. If ILockableEncryptor inherits from IEncryptor,
it will violate the Liskov Substitution Principle

monkey/monkey_island/cc/server_utils/encryption/i_lockable_encryptor.py

@@ -1,6 +1,4 @@
-from abc import abstractmethod
-
-from . import IEncryptor
+from abc import ABC, abstractmethod
 
 # NOTE: The ILockableEncryptor introduces temporal coupling, that is, you must first unlock the
 #       encryptor before you can use it. This is because the key material used to encrypt repository
@@ -33,7 +31,7 @@ class ResetKeyError(Exception):
     """
 
 
-class ILockableEncryptor(IEncryptor):
+class ILockableEncryptor(ABC):
     """
     An encryptor that can be locked or unlocked.