remove version from the Server header

while we still want to know which server is running to ease operation, the version was giving too much information on the installation, so let's remove it.

Author: benoitc

gunicorn/__init__.py

@@ -5,4 +5,4 @@
 
 version_info = (20, 0, 4)
 __version__ = ".".join([str(v) for v in version_info])
-SERVER_SOFTWARE = "gunicorn/%s" % __version__
+SERVER_SOFTWARE = "gunicorn"