Remove the "Cannot measure text in a removed node" error

Measuring text in a removed node produces erroneous results. Currently
we throw an error if this is attempted. However, erroring out plottable is
much worse than getting a bad text measurement on a removed node, since
probably when you add the node back in it will re-measure appropriately.
Merge pull request #624 from palantir/remove-removed-node-warning

Remove the "Cannot measure text in a removed node" error