Remove useless line

Signed-off-by: Thomas Daubney <thomas.daubney@arm.com>
diff --git a/scripts/generate_driver_wrappers.py b/scripts/generate_driver_wrappers.py
index 538999a..de89959 100755
--- a/scripts/generate_driver_wrappers.py
+++ b/scripts/generate_driver_wrappers.py
@@ -178,7 +178,6 @@
 
     repo_root = os.path.abspath(args.repo_root)
 
-    library_dir = ''
     if build_tree.looks_like_mbedtls_root(repo_root):
         library_dir = 'library'
     elif build_tree.looks_like_tf_psa_crypto_root(repo_root):