Don't attempt to plot gridspec layout ipymessages

[nstrayer]

This is a bandaid workaround for #4256

Fully fixing #4256 will take a good amount of investment. (It's worth noting that the mentioned example doesn't work in notebooks in vscode, either.)

This PR simply suppresses the final of the three messages that are produced by running leafmap.linked_maps(), which is one containing a gridspec widget that attempts to arrange the two previously emitted maps side-by-side.

Prior to this PR the result was three results in the plots pane, with the final one being empty.

After this PR two maps are shown in separate plots.

QA Notes

The following code when run in the console should at least do something and not look like something is loading forever. The behavior in the notebooks is not addressed here and does not work.

import leafmap
layers = ["ROADMAP", "HYBRID"]
leafmap.linked_maps(rows=1, cols=2, height="400px", layers=layers)