Move benchmarks to toplevel folder

python/asv.conf.json

@@ -48,7 +48,7 @@
 
     // The directory (relative to the current directory) that benchmarks are
     // stored in.  If not provided, defaults to "benchmarks"
-    "benchmark_dir": "pyarrow/benchmarks",
+    "benchmark_dir": "benchmarks",
 
     // The directory (relative to the current directory) to cache the Python
     // environments in.  If not provided, defaults to "env"