remove discontinued submodules if they are checked out

s/build

@@ -33,6 +33,7 @@ if [ ! -d $ROOT/$BUILD ]; then
   exit
 fi
 
+rm -rf crypto tolua
 git submodule update
 
 echo "build eressea"

s/preview

@@ -22,6 +22,7 @@ exit $2 # otherwise
 function build() {
 assert_dir $SOURCE
 cd $SOURCE
+rm -rf crypto tolua
 git fetch || abort "failed to update source. do you have local changes?"
 [ -z $1 ] || git checkout $1
 git pull