Add eslint rule @typescript-eslint/no-unnecessary-type-assertion

And fix all files accordingly.

Release-Notes: skip
Change-Id: I5a151b1d5399aabfe1550f5323e96a452f47cc7f
diff --git a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.ts b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.ts
index bfe396c..eff22d1 100644
--- a/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.ts
+++ b/polygerrit-ui/app/elements/settings/gr-change-table-editor/gr-change-table-editor.ts
@@ -81,12 +81,10 @@
    */
   _getDisplayedColumns() {
     if (this.root === null) return [];
-    return (
-      Array.from(
-        this.root.querySelectorAll(
-          '.checkboxContainer input:not([name=number])'
-        )
-      ) as HTMLInputElement[]
+    return Array.from(
+      this.root.querySelectorAll<HTMLInputElement>(
+        '.checkboxContainer input:not([name=number])'
+      )
     )
       .filter(checkbox => checkbox.checked)
       .map(checkbox => checkbox.name);