[release] package.json: remove 'preview' property

It was accidentally added during merge for v0.35.0

Fixes #2355
Fixes #2352

Change-Id: I50d4993fd2a6d2b84142c5db9ffc4a8aeda2c7cf
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/418194
Reviewed-by: Nooras Saba‎ <[email protected]>
TryBot-Result: kokoro <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah

package-lock.json

@@ -1,8 +1,7 @@
 {
   "name": "go",
   "displayName": "Go",
-  "version": "0.35.0",
-  "preview": true,
+  "version": "0.35.1",
   "publisher": "golang",
   "description": "Rich Go language support for Visual Studio Code",
   "author": {