Don't reassign crc on ChunkStream close

[radarhere]

Cherry-picking a commit from #2941

Pillow/src/PIL/PngImagePlugin.py

Lines 160 to 164 in bdbf59d

	class ChunkStream:
	def __init__(self, fp):
	self.fp = fp
	self.queue = []

Pillow/src/PIL/PngImagePlugin.py

Lines 191 to 192 in bdbf59d

	def close(self):
	self.queue = self.crc = self.fp = None

Ok, so when it is closing, it sets fp and queue to None, fair enough. What is crc?

Pillow/src/PIL/PngImagePlugin.py

Line 204 in bdbf59d

def crc(self, cid, data):

...it's a method? I don't see why this should be set to None.

#2935 shows that it used to be sometimes set to crc_skip, but that still doesn't explain it to me.

Looking at that PR, I conclude that

Pillow/src/PIL/PngImagePlugin.py

Lines 226 to 227 in bdbf59d

	def crc_skip(self, cid, data):
	"""Read checksum. Used if the C module is not present"""

is no longer correct, so I've also removed the second sentence in that docstring.