[SecuritySolution] Unskip fullscreen timeline tests (elastic#174593)

## Summary

Unskips the fullscreen timeline tests.

Looking at the issue described in
elastic#172547, it appears that the
timeline was somehow not opening correctly in some cases. This flakiness
has been removed in elastic#173413, so it
should be possible to unskip these tests.

In addition to unskipping the tests, an unnecessary task execution was
removed and two force clicks were cleaned up.

[Flaky test
runner](https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/4834)
(200/200)

fixes elastic#172547

x-pack/test/security_solution_cypress/cypress/e2e/investigations/timelines/full_screen.cy.ts

@@ -14,16 +14,14 @@ import {
   enterFullScreenMode,
   exitFullScreenMode,
 } from '../../../tasks/security_main';
-import { populateTimeline } from '../../../tasks/timeline';
 
 import { hostsUrl } from '../../../urls/navigation';
 
-describe.skip('Toggle full screen', { tags: ['@ess', '@serverless'] }, () => {
+describe('Toggle full screen', { tags: ['@ess', '@serverless'] }, () => {
   beforeEach(() => {
     login();
     visitWithTimeRange(hostsUrl('allHosts'));
     openTimelineUsingToggle();
-    populateTimeline();
   });
 
   it('Should hide timeline header and tab list area', () => {

x-pack/test/security_solution_cypress/cypress/tasks/security_main.ts

@@ -29,9 +29,9 @@ export const closeTimelineUsingCloseButton = () => {
 };
 
 export const enterFullScreenMode = () => {
-  cy.get(TIMELINE_FULL_SCREEN_BUTTON).first().click({ force: true });
+  cy.get(TIMELINE_FULL_SCREEN_BUTTON).first().click();
 };
 
 export const exitFullScreenMode = () => {
-  cy.get(TIMELINE_EXIT_FULL_SCREEN_BUTTON).first().click({ force: true });
+  cy.get(TIMELINE_EXIT_FULL_SCREEN_BUTTON).first().click();
 };