remove PR template (istio#2225)

Automatic merge from submit-queue.

Remove PR template

**What this PR does / why we need it**:

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes istio#2201 

**Special notes for your reviewer**:

**Release note**:

```release-note
none
```