Fix unused variable in generate_psa_tests.py

Remove the newly-unused variable that became unused in
a previous commit.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/tests/scripts/generate_psa_tests.py b/tests/scripts/generate_psa_tests.py
index 1e5d204..39fb210 100755
--- a/tests/scripts/generate_psa_tests.py
+++ b/tests/scripts/generate_psa_tests.py
@@ -714,7 +714,6 @@
         return
     # List in a cmake list format (i.e. ';'-separated)
     if options.list_for_cmake:
-        filenames = []
         print(';'.join(generator.filename_for(name)
                        for name in sorted(generator.TARGETS)), end='')
         return