Remove the libtensorflow_c.so target.

This was identical to libtensorflow.so for all but
a few days in the life of libtensorflow_c.so's existence.
Remove libtensorflow_c.so to remove any confusion about
which target to use for the TensorFlow C library.

(tensorflow@783c52e
and tensorflow#3830
were the changes that resulted in two targets doing the same thing)

I have made attempts to remove use of libtensorflow_c.so in
other projects such as:
LaurentMazare/tensorflow-ocaml#19
tensorflow/rust#56
malmaud/TensorFlow.jl#157
Change: 148665838

tensorflow/BUILD

@@ -298,15 +298,6 @@ cc_binary(
     ],
 )
 
-cc_binary(
-    name = "libtensorflow_c.so",
-    linkshared = 1,
-    deps = [
-        "//tensorflow/c:c_api",
-        "//tensorflow/core:tensorflow",
-    ],
-)
-
 cc_binary(
     name = "libtensorflow_cc.so",
     linkshared = 1,