Cleanup tp tubes a bit

[SX]

Remove:

pipeworks/teleport_tube.lua

Lines 124 to 136 in 8c25180

 -- we haven't found any tp tube to update, so lets add it 
 -- but sanity check that the hash has not already been inserted. 
 -- if so, complain very loudly and refuse the update so the player knows something is amiss. 
 -- to catch regressions of https://github.com/minetest-mods/pipeworks/issues/166 
 local existing = tp_tube_db[hash] 
 if  existing ~= nil then 
 	local e = "error" 
 	minetest.log(e, "pipeworks teleport tube update refused due to position hash collision") 
 	minetest.log(e, "collided hash: "..hash) 
 	minetest.log(e, "tried-to-place tube: "..fmt(pos)) 
 	minetest.log(e, "existing tube: "..fmt(existing)) 
 	return 
 end 

Also remove these everywhere and just use direct tp_tube_db, just load db during mod loading stage instead. No real reason for deferred db read:

pipeworks/teleport_tube.lua

Line 114 in 8c25180

 local tubes = tp_tube_db or read_tube_db() 

Tp tube table tp_tube_db is local to file, recreating local for function scope in most cases also isn't helping with performance but instead just add unnecessary LoC.