Remove use of deprecated OpenJPEG "bpp" member

[radarhere]

https://github.com/python-pillow/Pillow/actions/runs/4696665571/jobs/8326957733#step:6:116

src/libImaging/Jpeg2KEncode.c: In function ‘j2k_encode_entry’:
src/libImaging/Jpeg2KEncode.c:345:9: warning: ‘bpp’ is deprecated: Use prec instead [-Wdeprecated-declarations]
345 | image_params[n].bpp = bpp;

We are already using prec just before this, so bpp can just be removed.

Pillow/src/libImaging/Jpeg2KEncode.c

Lines 344 to 345 in 8740e32

	image_params[n].prec = prec;
	image_params[n].bpp = bpp;

See uclouvain/openjpeg#1383 for more information.