removed default reviewers (#369)

Author: gyliu513

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,10 +11,6 @@
 **Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
 Fixes #
 
-**Specify your PR reviewers**:
-<!-- Add or remove reviewers as your request -->
-/cc @gyliu513 @jichenjc @xunpan
-
 **Special notes for your reviewer**:
 
 /area provider/ibmcloud