Remove 'navigator' dependency from 'editor-preview'

Fixes eclipse-theia#6639

- removes the unused `navigator` from the `editor-preview` extension.
- the `navigator` was needlessly pulled by the `editor-preview` as it was
never used and is likely to be a leftover of 00e7d9d.

Signed-off-by: vince-fugnitto <vincent.fugnitto@ericsson.com>

packages/editor-preview/package.json

@@ -4,8 +4,7 @@
   "description": "Theia - Editor Preview Extension",
   "dependencies": {
     "@theia/core": "^0.12.0",
-    "@theia/editor": "^0.12.0",
-    "@theia/navigator": "^0.12.0"
+    "@theia/editor": "^0.12.0"
   },
   "publishConfig": {
     "access": "public"