Remove unused JSNAMESPACE_SUPPORT from event-dispatch

[rahatarmanahmed]

This was an old feature that let you add jsnamespace="ns to an element to implicitly add the ns. namespace to all unnamespaced JSAction handlers on or under that element. This required an expensive DOM walk to find the jsnamespace, and almost all JSAction users turned this feature off. The last usage in google 3was removed, so we can delete this now.

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• [ ] Bugfix
• [ ] Feature
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[rahatarmanahmed]

Caretaker note: cl/630174160 is needed either in sync CL or prior to sync CL due to copybara changes.

[rahatarmanahmed]

TESTED=TGP (failures are extremely flaky and fail at HEAD)

[AndrewKushnir]

Caretaker note: this PR is ready for merge, the only missing piece was an additional public-api approval, but since this is a removal of a no longer used feature, existing approvals should be sufficient.

[AndrewKushnir]

This PR was merged into the repository by commit 879537466ea1d8485599176f4a7cb2d0d81ac870.