Drop wheel from pyproject.toml examples

Per pypa/setuptools#3056 the addition of
wheel in build_system.requires is redundant since SetupTools already
drags it in anyway. Simplify our recommendations accordingly.

Change-Id: I0359ed11a6f0c54dc1af6a7f5f34e49476c58fb0

doc/source/user/using.rst

@@ -41,12 +41,12 @@ PBR can be configured as a PEP517 build-system in ``pyproject.toml``. This
 currently continues to rely on setuptools which means you need the above
 ``setup.py`` file to be present. The main benefits to using a
 ``pyproject.toml`` file with PBR are that you can control the versions of
-PBR, setuptools, and wheel that are used avoiding easy_install invocation.
+PBR and setuptools that are used avoiding easy_install invocation.
 Your build-system block in ``pyproject.toml`` will need to look something
 like this::
 
     [build-system]
-    requires = ["pbr>=5.7.0", "setuptools>=36.6.0", "wheel"]
+    requires = ["pbr>=5.7.0", "setuptools>=36.6.0"]
     build-backend = "pbr.build"
 
 

pyproject.toml.future

@@ -4,6 +4,6 @@
 # once we are more confident it works generally.
 
 [build-system]
-requires = ["setuptools>=36.6.0", "wheel"]
+requires = ["setuptools>=36.6.0"]
 build-backend = "pbr.build"
 backend-path = ["."]