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>

tests/scripts/generate_psa_tests.py

@@ -714,7 +714,6 @@ def main(args):
         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