Remove unused collapsible props from IDE

What does this MR do?

Working on a somewhat related MR (!26043 (closed)), I realized that the collapsible prop in resizable_panel.vue was always false and therefore never used. This led to a whole slew of removing unused things:

• 'rightPanelCollapsed' state
• 'leftPanelCollapsed' state
• 'setPanelCollapsedStatus' action
• 'SET_X_PANEL_COLLAPSED' mutation

But wait... If collapsible is unused, doesn’t it mean we can use it to make the left sidebar collapsible? We want that right?

I was hoping this was the case, but it soon became clear why we disabled this collapsing.

• The project header does not support being collapsed yet.
• The collapse uses a rightPaneCollapsed state which has been replaced by rightPane/isOpen a long time ago.

We're trying to move to a simpler and reusable abstraction between the left and right sidebar, but this old abstraction is causing some clutter and confusion.

Screenshots

Everything still works 😄

20200513_ide_everything_still_works