[release] package.json: do not claim .tmpl files in stable version

https://go-review.googlesource.com/c/vscode-go/+/281392 is a prep work
for template support. However, the handling is not available in gopls
yet, and LSP registration is activated only in the preview mode (per
https://go-review.googlesource.com/c/vscode-go/+/283639)

This CL is the remaining task from cl/283639 to disable the experimental
feature from the stable release. We will revert this change when gopls
is ready to handle templates.

Update #609

Change-Id: I72d04b11f14fde25b7ad64e6a9a82f7c20e32b4c
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/284095
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

package.json

@@ -131,15 +131,6 @@
         "aliases": [
           "Go Checksum File"
         ]
-      },
-      {
-        "id": "tmpl",
-        "extensions": [
-          ".tmpl"
-        ],
-        "aliases": [
-          "Go Template File"
-        ]
       }
     ],
     "grammars": [