If you go to Tools/ Customize/ Commands you will find it, or you can just rightclick in a free area over the drawing area and the Customize meny will open, and go from there, the Cuztoize can be placed different plazes depeending if what config you use, my examle is from a "Normal Metric"
Scroll down to Trim, select it and just drag it over to any toolbar you want, i did put it next to the "normal" Object Trim tool.
In the same Customize menu, go to options second from top is Workspace, click on the small arrov there, that will let you choose between Default or LTE.
How to use it
Select the 2 lines that is Your borders, click on the flag or rightclick and select Finish selection, then you can click in the drawing and drag a window that crossing the lines you want to trim between the cutting lines, then click finish one more time, and what you want happens
NB !! it have to be from right to left when you Select last time, else it will only trim the first one.
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB