boinc: fix location of /etc in $out

Looks like the latest version no longer requires the patch, and the patch instead resulted in
/etc files being installed to `$out/$out/etc` instead of `$out/etc`

pkgs/applications/science/misc/boinc/default.nix

···
     libxcb xcbutil
   ];
 
-  postConfigure = ''
-    sed -i -e s,/etc,$out/etc, client/scripts/Makefile
-  '';
-
   NIX_LDFLAGS = "-lX11";
 
   preConfigure = ''