test: Remove outdated comment

The serialize related methods were removed in commit
30007fda76.

If someone wants to see the tested methods, they can just read the test
itself, instead of relying on the wrong comment.

src/test/arith_uint256_tests.cpp

@@ -368,7 +368,7 @@ static bool almostEqual(double d1, double d2)
     return fabs(d1-d2) <= 4*fabs(d1)*std::numeric_limits<double>::epsilon();
 }
 
-BOOST_AUTO_TEST_CASE(methods) // GetHex operator= size() GetLow64 GetSerializeSize, Serialize, Unserialize
+BOOST_AUTO_TEST_CASE(methods)
 {
     BOOST_CHECK(R1L.GetHex() == R1L.ToString());
     BOOST_CHECK(R2L.GetHex() == R2L.ToString());