job_submit/lua: remove unused reference to 'posix' module

The sample code does not refer to the 'posix' Lua module. However, the
absence of the module is sufficient to keep slurmctld from starting, which is
confusing when trying to learn the capabilities of the job_submit/lua module.

Removing the inclusion does not hurt the code and makes it easier to get
started.  Patch from Gerrit.

contribs/lua/job_submit.lua

@@ -8,8 +8,6 @@
 
 --]]
 
-require "posix"
-
 function _build_part_table ( part_list )
 	local part_rec = {}
 	local i = 1
@@ -38,7 +36,7 @@ function slurm_job_submit ( job_desc, part_list )
 			 job_desc.user_id, account)
 		job_desc.account = account
 	end
---	If no default partition, the set the partition to the highest
+--	If no default partition, set the partition to the highest
 --	priority partition this user has access to
 	if job_desc.partition == nil then
 		local new_partition = nil