remove docs reference to deleted requirements/development.txt

Author: hodgesmr

docs/how-to/contributing/setting_up_the_environment.rst

@@ -6,18 +6,12 @@ Installing all dependencies
 
 All development dependencies can be installed by running::
 
-  $ pip install -r requirements/development.txt
+  $ pip install .[development]
 
 It is recommended to do this using a virtual environment tool of your choice.
 
 For example, when using the virtual environment library :code:`venv`::
 
-  $ python -m venv axelrod_development
-  $ source axelrod_development/bin/activate
-  $ pip install -r requirements/development.txt
-
-Alternatively, you can specify the development variant rather than the path::
-
   $ python -m venv axelrod_development
   $ source axelrod_development/bin/activate
   $ pip install .[development]