Remove unused variable

This was introduced when the role was created using os_cinder as
a starting point [1], as has never been used in the manila role.

[1] f9bfb7f0bc

Change-Id: I57df438cd25efd30e0e437470fc34df8fea1317c

defaults/main.yml

@@ -299,12 +299,6 @@ manila_api_paste_ini_overrides: {}
 manila_manila_conf_overrides: {}
 manila_api_uwsgi_ini_overrides: {}
 
-## Set default manila path in service units. The default override sets the
-## execution path for the manila service.
-manila_environment_overrides:
-  Service:
-    Environment: "PATH={{ manila_bin }}:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
-
 # Manila keypair
 #
 # The following path ontains the keypair which will be used for SSH.  It requires that