gh-149755: remove decimal benchmarks

They are integrated to the pyperformance, though not enabled yet, see
python/pyperformance#473.

Author: skirpichev

Modules/_decimal/tests/README.txt

@@ -2,9 +2,6 @@
 
 This directory contains extended tests and a benchmark against decimal.py:
 
-  bench.py  ->  Benchmark for small and large precisions.
-  Usage: ../../../python bench.py
-
   formathelper.py   ->
   randdec.py        ->  Generate test cases for deccheck.py.
   randfloat.py      ->