uninstalling dcsext breaks MissionScripting.lua

[jtoppins]

As part of the install process there is a modification to the MissionScripting.lua game file.

  --Initialization script for the Mission lua Environment (SSE)

  dofile('Scripts/ScriptingSystem.lua')
+ dofile(lfs.writedir()..'Scripts/loadplugins.lua')

  --Sanitize Mission Scripting environment
  --This makes unavailable some unsecure functions.
  --Mission downloaded from server to client may contain potentialy harmful lua code that may use these functions.

This should be changed to test for the existence of the file otherwise it can break the file.