screen/win32: remove 15-bit switch
We can't detect it with BITSPIXEL according to [1]:
When nIndex is BITSPIXEL and the device has 15bpp or 16bpp, the return value is 16.
There is no other usable field to tell the difference. But BI_RGB
in BITMAPINFOHEADER
corresponds to 15-bit usage, even for 16 bits bitcount.
[1] https://learn.microsoft.com/en-us/windows/win32/api/wingdi/nf-wingdi-getdevicecaps#return-value
Edited by Steve Lhomme