gnu: Remove python-flake8 native-input from some packages.

Remove python-flake8 from [native-inputs] in the following packages as
not required for build/tests/install:

* gnu/packages/bioinformatics.scm: (python-doubletdetection)
* gnu/packages/ebook.scm: (calibre)
* gnu/packages/graph.scm: (python-pygsp)
* gnu/packages/machine-learning.scm: (python-pymanopt, python-pyro-api)
* gnu/packages/python-web.scm: (python-furl)
* gnu/packages/python-xyz.scm: (python-jinja2-cli)
* gnu/packages/simulation.scm: (python-dolfin-adjoint)
* gnu/packages/vpn.scm: (sshuttle)

* gnu/packages/music.scm (python-mutagen):
  [arguments]<#:test-flags>: Ignore flake8 quality tests.
  [native-inputs]: Remove python-flake8.

Change-Id: I2789b8711faf3aa60a4bfa2062f95616e216d6c3
Signed-off-by: Sharlatan Hellseher <sharlatanus@gmail.com>

gnu/packages/machine-learning.scm

@@ -1597,7 +1597,6 @@ learning.")
                ((",!=1.12\\.\\*") "")))))))
     (propagated-inputs (list python-numpy python-scipy))
     (native-inputs (list python-autograd
-                         python-flake8
                          python-flake8-bugbear
                          python-isort
                          python-matplotlib
@@ -6465,8 +6464,7 @@ and Numpy.")
     (build-system pyproject-build-system)
     (arguments '(#:tests? #false)) ;requires pyro
     (native-inputs
-     (list python-flake8
-           python-ipython
+     (list python-ipython
            python-pytest
            python-setuptools
            python-sphinx