ci: drop --ignore-optional flag from yarn

Fixes image compression step:
```
Starting 'images'...
gulp-imagemin: Couldn't load default plugin "optipng"
'images' errored after 23 ms
  TypeError in plugin "gulp-imagemin"
Message:
    fn is not a function
Details:
    fileName: /home/jenkins/workspace/website/dev.vac.dev/assets/img/arrow-peach.svg
    domainEmitter: [object Object]
    domainThrown: false
```

Signed-off-by: Jakub Sokołowski <jakub@status.im>

Jenkinsfile

@@ -32,7 +32,7 @@ pipeline {
 
     stage('Install Deps') {
       steps {
-        sh 'yarn install --ignore-optional'
+        sh 'yarn install'
         sh 'bundle install'
       }
     }