Remove all uses of `fw-icon` class and clean up unused legacy icon classes

[nwmac]

Follow on from #15130

The fw-icon class is used in these places:

• shell/components/LocaleSelector.vue
• shell/components/form/ProjectMemberEditor.vue
• shell/components/form/Members/ClusterPermissionsEditor.vue
• shell/components/nav/WindowManager/index.vue

We should remove it from there and also remove some of the other icon classes that are defined but not used.

[yonasberhe23]

The bot failed to reopen this issue