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:

docker-tools/eng/common/templates/steps/validate-image-sizes.yml

Line 16 in 4a49a26

displayName: Run Image Size Tests

docker-tools/src/Microsoft.DotNet.ImageBuilder/src/Commands/ImageSizeCommand.cs

Line 17 in 4a49a26

public abstract class ImageSizeCommand<TOptions, TOptionsBuilder> : ManifestCommand<TOptions, TOptionsBuilder>

[dotnet-issue-labeler]

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.

[dotnet-issue-labeler]

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.