Cleanup tp tubes a bit

[S-S-X]

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.