Remove unused tick_formats file

echarts superseded the code here, so it can be removed

app/assets/javascripts/lib/utils/tick_formats.js

-import { createDateTimeFormat } from '../../locale';
-
-let dateTimeFormats;
-
-export const initDateFormats = () => {
-  const dayFormat = createDateTimeFormat({ month: 'short', day: 'numeric' });
-  const monthFormat = createDateTimeFormat({ month: 'long' });
-  const yearFormat = createDateTimeFormat({ year: 'numeric' });
-
-  dateTimeFormats = {
-    dayFormat,
-    monthFormat,
-    yearFormat,
-  };
-};
-
-initDateFormats();
-
-/**
-  Formats a localized date in way that it can be used for d3.js axis.tickFormat().
-
-  That is, it displays
-  - 4-digit for first of January
-  - full month name for first of every month
-  - day and abbreviated month otherwise
-
-  see also https://github.com/d3/d3-3.x-api-reference/blob/master/SVG-Axes.md#tickFormat
-  */
-export const dateTickFormat = date => {
-  if (date.getDate() !== 1) {
-    return dateTimeFormats.dayFormat.format(date);
-  }
-
-  if (date.getMonth() > 0) {
-    return dateTimeFormats.monthFormat.format(date);
-  }
-
-  return dateTimeFormats.yearFormat.format(date);
-};

spec/javascripts/lib/utils/tick_formats_spec.js

-import { dateTickFormat, initDateFormats } from '~/lib/utils/tick_formats';
-
-import { setLanguage } from '../../helpers/locale_helper';
-
-describe('tick formats', () => {
-  describe('dateTickFormat', () => {
-    beforeAll(() => {
-      setLanguage('de');
-      initDateFormats();
-    });
-
-    afterAll(() => {
-      setLanguage(null);
-    });
-
-    it('returns year for first of January', () => {
-      const tick = dateTickFormat(new Date('2001-01-01'));
-
-      expect(tick).toBe('2001');
-    });
-
-    it('returns month for first of February', () => {
-      const tick = dateTickFormat(new Date('2001-02-01'));
-
-      expect(tick).toBe('Februar');
-    });
-
-    it('returns day and month for second of February', () => {
-      const tick = dateTickFormat(new Date('2001-02-02'));
-
-      expect(tick).toBe('2. Feb.');
-    });
-
-    it('ignores time', () => {
-      const tick = dateTickFormat(new Date('2001-02-02 12:34:56'));
-
-      expect(tick).toBe('2. Feb.');
-    });
-  });
-});