Unlike removing a layer from a project, deleting a layer deletes it from the layer library permanently.
It is best to only delete layers that are either out of date, no longer viable, or were mistakenly uploaded.
From a Project
Open the 3-dot menu next to the layer
Select "Manage layer"
Click the Bin to delete the layer.
Confirm delete:
Cancel - return to menu without deleting
Delete - permanently delete the layer from the library and return to the project
From the Layer Library
Open the layer library.
Search for the layer you want to delete.
Click More on the far right
Click the Cog to open layer settings
Click the Bin to delete the layer.
Confirm delete:
Cancel - return to menu without deleting
Delete - permanently delete the layer from the library and return to the project