mgr/dashboard: remove TSlint no-unused-variable rule

to silence warning like

no-unused-variable is deprecated. Since TypeScript 2.9. Please use the
built-in compiler checks instead.

we are using TypeScript 3.1.3 at the time of writing.

see palantir/tslint#3919

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/pybind/mgr/dashboard/frontend/tslint.json

@@ -40,7 +40,6 @@
     "no-trailing-whitespace": true,
     "no-unnecessary-initializer": true,
     "no-unused-expression": true,
-    "no-unused-variable": true,
     "no-use-before-declare": true,
     "no-var-keyword": true,
     "object-literal-sort-keys": false,