Fix placing certain signs

[tenplus1]

This fixes #1 by removing node extensions before placing, just incase player got a hold of certain sign nodes they shouldnt have.

[OgelGames]

Alternatively, you could remove the after_place_node function from the other sign nodes here, after the table gets copied:

signs_lib/api.lua

Line 1088 in 3d5ba32

local othermounts_def = table.copy(def)

othermounts_def.after_place_node = nil

[tenplus1]

@OgelGames - Tried that, gives an error, and using an empty function has sign placement issues.

[SwissalpS]

Maybe this change to MT helps in future versions
minetest/minetest@408faa1