[Clang][perf-training] Fix clean command in perf-helper.py (#118978)

The first path argument was always being ignored, and since most calls
to this command only passed one path, it wasn't actually doing anything
in most cases.

This bug was introduced by dd0356d741aefa25ece973d6cc4b55dcb73b84b4.

(cherry picked from commit 18af3fc1bf8855e1e166e64a9210ed07d610aa54)

clang/utils/perf-training/perf-helper.py

@@ -36,7 +36,7 @@ def clean(args):
             + "\tRemoves all files with extension from <path>."
         )
         return 1
-    for path in args[1:-1]:
+    for path in args[0:-1]:
         for filename in findFilesWithExtension(path, args[-1]):
             os.remove(filename)
     return 0