Do not treat RGB as a rawmode for an RGB palette

[radarhere]

At the moment, when creating a raw ImagePalette with an RGB rawmode,

>>> from PIL import ImagePalette
>>> palette = ImagePalette.raw("RGB", [0, 1, 2])
>>> palette.tobytes()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/ImagePalette.py", line 124, in tobytes
    raise ValueError(msg)
ValueError: palette contains raw palette data

Oh, I'll have to do something so that this 'raw palette data' can be decoded into... wait a minute.

>>> palette.mode
'RGB'

The rawmode and mode are the same! Who needs to decode anything?

ImagePalette.raw() creates an ImagePalette in an RGB mode.

Pillow/src/PIL/ImagePalette.py

Lines 234 to 235 in 7e57b6c

	def raw(rawmode: str, data: Sequence[int] | bytes | bytearray) -> ImagePalette:
	palette = ImagePalette()

Pillow/src/PIL/ImagePalette.py

Lines 46 to 48 in 7e57b6c

	def __init__(
	self,
	mode: str = "RGB",

If the rawmode is also in RGB mode, then instead of setting rawmode on the palette and potentially raising an error later, let's simply not set rawmode.