gstlal-inspiral/python/dags/layers/inspiral.py: move "injections" option to common_opts

The injections option is not given a file for the offline analysis (that's the "injection-file"), resulting in an input which is NoneType. This causes an error when the analysis tries to iterate through the input. This bugfix moves the injections argument out of the input options and into the common options, where it will only be interpreted as a flag for the offline analysis.