Remove deprecated functionality

[edgarrmondragon]

1. Support for nullary jitter functions

   backoff/backoff/_common.py

   Lines 33 to 42 in 1bdab90

   	try:
   	seconds = jitter(value) if jitter is not None else value
   	except TypeError:
   	warnings.warn(
   	"Nullary jitter function signature is deprecated. Use "
   	"unary signature accepting a wait value in seconds and "
   	"returning a jittered version of it.",
   	DeprecationWarning,
   	stacklevel=2,
   	)