To be even more specific I would use these selectors to select the left sidebar button, and the downwards arrow within the root window:
.mod-root .workspace-tab-header-tab-list > span,
.sidebar-toggle-button.mod-left {
display: none
}
Without the .mod-root some arrows in the left side panel would possibly be affected. And without the .mod-left also the right sidebar button will not show.