Remove --system-site-packages argument to ./configure.

MediaGoblin ignores this argument and creates a virtualenv with
--system-site-packages regardless. Appears it's been this way since the early
days.

Removing this make the installation instructions easier to read.

docs/source/siteadmin/deploying.rst

@@ -256,7 +256,7 @@ Clone the MediaGoblin repository and set up the git submodules::
 Set up the environment::
 
     $ ./bootstrap.sh
-    $ VIRTUALENV_FLAGS='--system-site-packages' ./configure
+    $ ./configure
     $ make
 
 Create and set the proper permissions on the ``user_dev`` directory.

docs/source/siteadmin/relnotes.rst

@@ -44,6 +44,8 @@ This chapter has important information about our current and previous releases.
 - Remove Debian 10 development Dockerfile (BenSturmfels)
 - Document running multiple MediaGoblin instances on one server (Ben Sturmfels)
 - Begin conversion from jQuery to vanilla JS (Ben Sturmfels)
+- Remove `--system-site-packages` from docs and CI builds as it is forced anyway
+  (Ben Sturmfels)
 
 **Bug fixes:**
 

docs/source/siteadmin/upgrading.rst

@@ -42,7 +42,7 @@ Upgrade
 
 4. Recreate the virtual environment and install MediaGoblin::
 
-     ./bootstrap.sh && VIRTUALENV_FLAGS='--system-site-packages' ./configure &&
+     ./bootstrap.sh && ./configure &&
      make
 
 5. Re-install any ":doc:`plugins`" you had previously installed. Skipping these