Remove "matching '...'" part from the pytest.raises message

When a test with pytest.raises(ValueError, match='foo') doesn't raise, the
following error is printed:

    Failed: DID NOT RAISE <class 'ValueError'> matching 'foo'

This error message is confusing as it implies a ValueError was raised, but the
message wasn't matching 'foo'.

I first considered rewording it somehow to preserve the match pattern in it, but
I don't think that's worthwhile as the pattern should usually be apparent from
the stacktrace anyways (hard-coded, as parametrization, or with --showlocals for
more sophisticated cases).

_pytest/python_api.py

@@ -571,7 +571,6 @@ def raises(expected_exception, *args, **kwargs):
             message = kwargs.pop("message")
         if "match" in kwargs:
             match_expr = kwargs.pop("match")
-            message += " matching '{0}'".format(match_expr)
         return RaisesContext(expected_exception, message, match_expr)
     elif isinstance(args[0], str):
         code, = args