Remove the template for Az 4.0 preview

[dingmeng-xue]

Description

Since we released Az 4.1 using current hybrid mode, I prefer to deleting original 4.0 preview issue template because it may make user use this template by mistake when they hit issue on Az 4.x

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
  • the markdown help files have been regenerated using the commands listed here

[adxsdkps]

Can one of the admins verify this patch?

[dcaro]

I have updated the http://aka.ms/azps4issue link accordingly.