doc: remove duplicate page inclusion

doc/usage/index.rst in branch origin/next includes usage/environment twice.
Remove the duplicate entry.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 3905540..33761af 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -11,7 +11,6 @@
    netconsole
    partitions
    cmdline
-   environment
 
 Shell commands
 --------------