Removing Docker Preview Task

[vincent1173]

Removing Docker Preview Task from vsts-tasks. It is available as an extension.

Changes made as per https://github.com/Microsoft/vsts-tasks/pull/1289

[msftclas]

Hi @vincentdass, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

It looks like you're a Microsoft contributor (Vincent A). If you're full-time, we DON'T require a Contribution License Agreement. If you are a vendor, please DO sign the electronic Contribution License Agreement. It will take 2 minutes and there's no faxing! https://cla.microsoft.com.

TTYL, MSBOT;

[KrishnaAdityaB]

Looks good. Go ahead and merge after running tests - https://github.com/Microsoft/vsts-tasks/blob/master/docs/contribute.md

[vincent1173]

All tests passed (tested using Gulp).
Ready to merge.