Remove unused PNG _OUTMODES

[radarhere]

Pillow/src/PIL/PngImagePlugin.py

Lines 1106 to 1122 in 767a3f1

	_OUTMODES = {
	# supported PIL modes, and corresponding rawmode, bit depth and color type
	"1": ("1", b"\x01", b"\x00"),
	"L;1": ("L;1", b"\x01", b"\x00"),
	"L;2": ("L;2", b"\x02", b"\x00"),
	"L;4": ("L;4", b"\x04", b"\x00"),
	"L": ("L", b"\x08", b"\x00"),
	"LA": ("LA", b"\x08", b"\x04"),
	"I;16": ("I;16B", b"\x10", b"\x00"),
	"I;16B": ("I;16B", b"\x10", b"\x00"),
	"P;1": ("P;1", b"\x01", b"\x03"),
	"P;2": ("P;2", b"\x02", b"\x03"),
	"P;4": ("P;4", b"\x04", b"\x03"),
	"P": ("P", b"\x08", b"\x03"),
	"RGB": ("RGB", b"\x08", b"\x02"),
	"RGBA": ("RGBA", b"\x08", b"\x06"),
	}

L;1, L;2, L;4, P;1, P;2 and P;4 are not valid Pillow modes. The P mode variants are created by

Pillow/src/PIL/PngImagePlugin.py

Line 1360 in 767a3f1

if mode == "P":

Pillow/src/PIL/PngImagePlugin.py

Line 1380 in 767a3f1

outmode += f";{bits}"

but the L mode variants are never used, and can be removed.