Disable Disc and ROM pipeline stages for the time being

We're still some way out from having these jobs pass - estimated at least 70
MRs until ROMs cross-compile. In the meantime, the main effect of these jobs
will be to cause the overall pipeline status to be amber (i.e. passed with
warnings) which undermines its usefulness as an at-a-glance indicator of
whether the other jobs all passed.

In theory, their job logs could be inspected by human reviewers to check
that the MR doesn't make a cross-compile build any worse than it already is,
but I'm not sure that this is going to be the case in practice, or whether
this overrides the benefit of beng able to quickly see that the whole
pipeline passes.

So, comment out the lines that generate that part of the YAML for the time
being. Reinstate them once the respective cross-compile builds can complete.
1 job for RISC_OSLib in 4 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #35564