Removed PPM loop to read header tokens

[radarhere]

PpmImagePlugin is structured to read each of the header tokens one at a time, and then perform an action with it.

Pillow/src/PIL/PpmImagePlugin.py

Lines 112 to 114 in a786a05

	for ix in range(3):
	token = int(self._read_token())
	if ix == 0: # token is the x size

It's simpler and shorter to remove the loop.