stbt.press_and_wait: Refactor: Remove workaround for 3-channel pixel_bounding_box
stbt.press_and_wait: Refactor: Remove workaround for 3-channel pixel_bounding_box

Not necessary since 733d82ad7 (_stbt.imgutils.pixel_bounding_box:
Support 3-channel images).

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester