Rather than open-coding it twice, use a shared routine for
deleting columns from the Settings structure.  In order to
improve code readability refactor the screen array deletion
similarly and call both routines from Settings_delete.

Be more flexible and consistent with the way other structures
are initialised by passing a pointer to struct Machine into the
Settings constructor.  This would allow other alternative setup
of default Meters in the future, e.g. for some nvtop-alike tool
sharing code with htop (hypothetically) showing GPUs instead of
CPUs by default.  Mainly it just makes the code easier to read.