Use existing palette mode for C getpalette mode

[radarhere]

Currently, C's getpalette() accepts a mode and a rawmode. If the mode isn't provided, it defaults to RGB.

Pillow/src/_imaging.c

Lines 1312 to 1319 in c61e26c

	char *mode_name = "RGB";
	char *rawmode_name = NULL;
	if (!PyArg_ParseTuple(args, "|ss", &mode_name, &rawmode_name)) {
	return NULL;
	}
	if (rawmode_name == NULL) {
	rawmode_name = mode_name;
	}

However, C already knows what mode the palette is, and why would we ever want to pack the palette data from another mode?

So let's change getpalette() to only accept a rawmode, that defaults to the current palette mode.