Remove ERROR_DUPLICATE_FILE error from strings_english.txt

This was originally removed by 08a3d4fc2b0e620d696649e2accfb8841bb753f3

A recent commit by grangeway d679a60b136c0e2522c40c10bc436c9b5f5c47de
reintroduced this string, but since the constant does not exist it
generates a system notice which prevents MantisBT from functioning
properly (impossible to login).