fix: removed deprecated dependency tensorflow-gpu

[dchebakov]

After this commit https://github.com/tensorflow/tensorflow/commit/2105f7fb148035b9198f278d702e14bbc821a526 in the Tensorflow repository, the dependency tensorflow-gpu isn't available for installation anymore

If you will try to build tensorflow/serving from master now, you will get the following error (which is fixed by this commit):

  × Running setup.py install for tensorflow-gpu did not run successfully.
  │ exit code: 1
  ╰─> [18 lines of output]
      Traceback (most recent call last):
        File "<string>", line 36, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/tmp/pip-install-rur6eq36/tensorflow-gpu_252849d94ab24c16b348e87779089fbf/setup.py", line 37, in <module>
          raise Exception(TF_REMOVAL_WARNING)
      Exception:
      
      =========================================================
      The "tensorflow-gpu" package has been removed!
      
      Please install "tensorflow" instead.
      
      Other than the name, the two packages have been identical
      since TensorFlow 2.1, or roughly since Sep 2019. For more
      information, see: pypi.org/project/tensorflow-gpu
      =========================================================