Improved Remove Background types

[RunDevelopment]

Changes:

• I inlined the removeBackground function because it was only used in one place.
• The main image output now guarantees an RGBA image. This wasn't enforced at runtime before.
• The image input now requires an RGB or RGBA image. Grayscale images are invalid inputs and cause a crash.

Open question: Should the image input accept RGBA images in the first place? RGBA images will be converted to RGB internally, so the alpha channel is completely ignored.