Fix a compilation problem - type PeekPokeTester not found (freechipsproject#144)

Author: lizhirui

2.3_control_flow.ipynb

@@ -456,10 +456,6 @@
     "  io.fOfX := result  \n",
     "}\n",
     "\n",
-    "// verify that the computation is correct\n",
-    "class PolynomialTester(c: Polynomial) extends PeekPokeTester(c) {\n",
-    "}\n",
-    "\n",
     "// Test Polynomial\n",
     "test(new Polynomial) { c =>\n",
     "  for(x <- 0 to 20) {\n",