BuildSystem: Remove unused variables.

Because of a copy-and-paste from the script build_tools/download_externals,
the script build_tools/list_valid_installed_externals got its local variables.
However in the latter, three variables were not used actually.

Change-Id: I252de5a98c17ea54459174875357c22c2eebe8d5

build_tools/list_valid_installed_externals

@@ -19,10 +19,7 @@ EOF
 
 XMLSTARLET=`getvar XMLSTARLET`
 ASTMODDIR=`getvar ASTMODDIR`
-cache_dir=`getvar EXTERNALS_CACHE_DIR`
-DOWNLOAD_TO_STDOUT=`getvar DOWNLOAD_TO_STDOUT`
 HOST_CPU=`getvar HOST_CPU`
-INSTALL=`getvar INSTALL`
 
 tmpdir=$(mktemp -d)
 if [[ -z "${tmpdir}" ]] ; then