Fix linting warnings for all no-unused-vars (#1913)

## Which problem is this PR solving?
- Resolves part of no-unused-vars warnings for #1608 

## Description of the changes
-  Unused variables have been removed

## How was this change tested?
- By running "yarn lint" command

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [x] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `yarn lint` and `yarn test`

packages/jaeger-ui/src/components/SearchTracePage/SearchForm.test.js

@@ -54,8 +54,6 @@ function makeDateParams(dateOffset = 0) {
   };
 }
 
-const DATE_FORMAT = 'YYYY-MM-DD';
-const TIME_FORMAT = 'HH:mm';
 const defaultProps = {
   dataCenters: ['dc1'],
   handleSubmit: () => {},

packages/jaeger-ui/src/components/TracePage/TracePageHeader/AltViewOptions.test.js

@@ -15,7 +15,6 @@
 import * as React from 'react';
 import { shallow } from 'enzyme';
 import { Dropdown } from 'antd';
-import { Link } from 'react-router-dom';
 import AltViewOptions from './AltViewOptions';
 import * as track from './TracePageHeader.track';
 import { ETraceViewType } from '../types';

packages/jaeger-ui/src/components/TracePage/TraceTimelineViewer/VirtualizedTraceView.tsx

@@ -372,7 +372,7 @@ export class VirtualizedTraceViewImpl extends React.Component<VirtualizedTraceVi
   };
 
   renderSpanBarRow(span: Span, spanIndex: number, key: string, style: React.CSSProperties, attrs: {}) {
-    const { spanID, childSpanIds } = span;
+    const { spanID } = span;
     const { serviceName } = span.process;
     const {
       childrenHiddenIDs,