compat: remove deprecated VirtualSize

[inknos]

Since compat version 1.43 the VirtualSize field in the GET /images/{name}/json, GET /images/json, and
GET /system/df responses is deprecated and will no longer be included in API v1.44. Use the Size field instead, which contains the same information.

Does this PR introduce a user-facing change?

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[inknos]

@mheon @Luap99 this is probably the most trivial PR for all docker api changes, and was not tested. should we add tests for this endpoint or mark it to be skipped?

[Luap99]

What happens if a client talks in an older API version? Should there be some version check around this instead?

[inknos]

I think the answer depends on our policy for deprecations. What is it? And is the libpod deprecation policy the same as for compat?

[Luap99]

I think the policy for compat is match whatever docker is doing. If you can run the latest docker and use API v1.40 (or anything below the removal) and it still returns that field then we need to do as well.

For libpod we have a strict semver commitment so any breaking changes such as remove response field should wait for the next major version.