Fix NSIS auto-uninstall

The NSIS script incorrectly passes the default installation dir as
argument for the auto-uninstaller of previous installs.

This is supposed to avoid moving the uninstaller to a temp file, but it
is unclear why that was done, as this leaves the uninstaller behind.

More importantly, now that we have changed the default installation
directory, the auto-uninstall tries to remove the previous install from
the new location, which does nothing and leaves behind several GB on the
disk.

source/tools/dist/0ad.nsi

@@ -229,7 +229,7 @@ Function .onInit
 ;Run the uninstaller
 uninst:
   ClearErrors
-  ExecWait '$R0 _?=$INSTDIR' ;Do not copy the uninstaller to a temp file
+  ExecWait '$R0'
 
 done: