28. (Machines only) Remove duplicate hardcoded snap info

Remove information that is hardcoded in the charm code and that duplicates the snap section of refresh_versions.toml.

Example refresh_versions.toml snap section

[snap]
name = "charmed-postgresql" (1)

[snap.revisions] (2)
# amd64
x86_64 = "182"
# arm64
aarch64 = "181"

1	Snap name
2	Snap revisions

Snap name

Replace the hardcoded snap name (e.g. "charmed-postgresql") with:

Replace hardcoded snap revision(s) (e.g. "182", "181") with:

the CharmSpecific.refresh_snap method’s snap_revision parameter or
charm_refresh.Machines().pinned_snap_revision

pinned_snap_revision must only be used during the initial snap installation and must not be used to refresh the snap