misc/notepadplus: add missing delete keyword

misc/notepadplus: add missing delete keyword

In general the description should describe what is added or fixed, not how it was done (the code does this), but in this case the cause was delete was missing, so the fix is to add it.

Fixes #5759.

R=adg
CC=dave, dominik.honnef, gobot, golang-dev
https://golang.org/cl/10476043