Getting Started:
Step 1:
Click the collection on the left pane.
Step 2:
Select the items(s) you want to remove.
Step 3:
Press Delete Key
or Right-click and choose Remove item(s) from Collections...
This will only removes the item(s) from that specific Collection, not from your entire Library.
Step 4:
Press Ctrl + Delete
or Right-click on the item and select Move Item(s) to Trash...
This will send Items to trash directly when a collection is selected.