Drop column transport_params from the Computer database model

[sphuber]

Fixes #2927

The column was intended to contain customized parameters to be used to
configure the transport of a computer. However, these fit better in the
AuthInfo, as these settings can change per user. The design is to keep
the Computer as generic as possible as to not be user dependent, such
that its configuration can be easily shared and reused. Generic computer
configuration can go in the metadata column. User specific settings
need to go in the AuthInfo.metadata instead.

[sphuber]

@giovannipizzi note that I built this on top of commits that are waiting review in PR #2944
The migrations here simply drop the column. Are we sure we don't need a data migration? Do we know if people had been putting data in transport_params. And if so, is there any way to deterministically migrate it? Where should it go? Computer.metadata or Authinfo.metadata, and in the case of the latter, which ones?

[coveralls]

Coverage decreased (-0.04%) to 72.594% when pulling 62978e0 on sphuber:fix_2927_drop_computer_transport_params into 6588683 on aiidateam:develop.

[giovannipizzi]

From the current code, do I understand correctly that there is no direct way for the user to set these from the CLI? If this is the case, I wouldn't be worried (are the prepend/append_text going into the metadata, right?) If instead these are settable from the command line then we must be more careful.

In the first case, if we really want to be sure, we can check if they are empty, and print them in case on file with warnings similar to when we dropped DbWorkflow etc.

[sphuber]

@giovannipizzi you are right, it was not exposed in the CLI and all relevant data was stored in the metadata. I would say we can merge it as is