[cleanup] Remove legacy preview instructions (apache#19380)

.github/PULL_REQUEST_TEMPLATE.md

@@ -75,7 +75,7 @@ This change added tests and can be verified as follows:
 
 <!-- DO NOT REMOVE THIS SECTION. CHECK THE PROPER BOX ONLY. -->
 
-- [ ] `doc` <!-- Your PR contains doc changes. Please attach the local preview screenshots (run `sh start.sh` at `pulsar/site2/website`) to your PR description, or else your PR might not get merged. -->
+- [ ] `doc` <!-- Your PR contains doc changes. -->
 - [ ] `doc-required` <!-- Your PR changes impact docs and you will update later -->
 - [ ] `doc-not-needed` <!-- Your PR changes do not impact docs -->
 - [ ] `doc-complete` <!-- Docs have been already added -->