Allow the removal of keyboard shortcuts

[elexis]

Problem: If one uses Alt+DoubleClick to select all units of the same kind, WASD to scroll and also experiences #2809, it is likely that the shortcut Alt+D = developer console will be triggered and people think that one cheats.

Thats why I decided to delete this shortcut by assigning it to an empty string in local.cfg. (​http://trac.wildfiregames.com/wiki/Manual_Settings)

With this config a the following error message is triggered unnecessarily:

ERROR: Encountered config setting 'hotkey.session.devcommands.toggle' without value while parsing 'config/local.cfg' on line 23

This patch removes the error message.

The error message is not necessary in this case, since the game builds and runs without problems and the shortcut is deleted successfully.