Use existing properties to set the max memory, don't add them to the resources

[giovannipizzi]

Instead of defining a new memory_Mb in the resources (very specific to this scheduler), we should reuse concepts already into AiiDA and independent of the scheduler.

E.g. we have metadata.options.max_memory_kb in CalcJobs:

https://github.com/aiidateam/aiida-core/blob/ff1318b485a8b803e115b78946cc4593fc661153/aiida/engine/processes/calcjobs/calcjob.py#L249

and this is passed to the scheduler in the JobTemplate:

https://github.com/aiidateam/aiida-core/blob/ff1318b485a8b803e115b78946cc4593fc661153/aiida/schedulers/datastructures.py#L284

See e.g. how it's used in SLURM:

https://github.com/aiidateam/aiida-core/blob/develop/aiida/schedulers/plugins/slurm.py#L383-L396

[sphuber]

Just as a note that we just merged aiidateam/aiida-core#5260 which adds the property default_memory_per_machine, which allows to define the default memory (in kB) to use for a particular Computer. We explicitly chose for kB (instead of MB) to be consistent with the already existing max_memory_kb option.