Revert "stick to MUXES_TO_PROCESS"

This reverts commit b721f6c757635e617ee8668505f2b640bade6f50.
Revert "add MUXES_TO_PROCESS environment variable (array)"

This reverts commit 85cc17ce1f96880d6b30963fa9f4b4ce9fe8ec3b.
Revert "rename N to SUBCAMERA_COUNT"

This reverts commit 616dbbed8761798831b77de779ed2decc6b83665.
Revert "allow to set environment variables from command line (warning: values could be overriden with /etc/defaults/rawdata-downloader ones)"

This reverts commit c8278313c93f808ec75fb0681cd9f23090f96c16.