Drop support for cancellable as a positional argument

Better to be explicit for the rare cases where a single operation needs
a specific Cancellable.

frida/core.py

@@ -18,12 +18,6 @@ Cancellable = _frida.Cancellable
 def cancellable(f):
     @wraps(f)
     def wrapper(*args, cancellable=None, **kwargs):
-        if cancellable is None:
-            last_arg = args[-1]
-            if isinstance(last_arg, Cancellable):
-                cancellable = last_arg
-                args = args[:-1]
-
         if cancellable is None:
             return f(*args, **kwargs)