[Python SDK] Fix get_image inferring PCD mime type

In GetImage in src/viam/components/camera/service.py (here), we guess the mime type → PCD when none is provided. GetImage shouldn’t deal with PCD at all.

Mirror RDK https://github.com/viamrobotics/rdk/blob/af53c676de1fc215fdad59038536f8cafc4881eb/components/camera/server.go#L54