Remove Image Size commands and pipeline code

[lbussell]

This issue is a follow-up to #745.

Once #745 is completed, the ImageSizeCommand and Image Size pipeline should be removed:

https://github.com/dotnet/docker-tools/blob/4a49a262d4084c9e09823e51248dabeb0317fb6f/eng/common/templates/steps/validate-image-sizes.yml#L16

https://github.com/dotnet/docker-tools/blob/4a49a262d4084c9e09823e51248dabeb0317fb6f/src/Microsoft.DotNet.ImageBuilder/src/Commands/ImageSizeCommand.cs#L17

[ghost]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.