Remove testonly attribute that breaks builds with newer bazel versions.

In bazel versions beyond 0.3.2, the testonly
attribute will be more strongly enforced
(5f026f9611)
and builds will fail with something like:

in cc_library rule //tensorflow/python:tf_session_helper: non-test target
'//tensorflow/python:tf_session_helper' depends on testonly target
'//tensorflow/python:construction_fails_op' and doesn't have testonly attribute
set.

Ideally, tf_session_helper wouldn't depend on :construction_fails_op
(and construction_fails_op wouldn't make its way into
//tensorflow/python:pywrap_tensorflow). Instead only the
py_test target //tensorflow/python:session_test would.
However, that results in errors like:

in deps attribute of py_test rule //tensorflow/python:session:test:
'//tensorflow/python:construction_fails_op' does not have mandatory
provider 'py'.

I believe the correct fix involves some reengineering of bazel
and/or the skylark rules we use. I'm punting on that for now:
(a) to expedite fixing #5143
(b) because this change is effectively a no-op since in bazel versions
    <= 0.3.2 were ignoring testonly altogether
    (https://github.com/bazelbuild/bazel/issues/1923)
Change: 137281711

tensorflow/python/BUILD

@@ -1931,7 +1931,6 @@ py_library(
 # Just used by tests.
 tf_cuda_library(
     name = "construction_fails_op",
-    testonly = 1,
     srcs = ["client/test_construction_fails_op.cc"],
     deps = [
         "//tensorflow/core",