[Clang][perf-training] Fix clean command in perf-helper.py (llvm#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 dd0356d.

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