The tool window (that happens to be on the left side bar) is called "Project" (quite often referred as "Project View" as well).
The actual highlighted area is not a toolbar -- it's a "toolwindow title bar" -- it cannot be disabled/hidden.
Most of tool windows do have actual toolbars (e.g. "Structure", "TODO", "Favourites", "Hierarchy" and many others) but only some of them can have it hidden (e.g. "Structure").
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