ilua: eliminate the "_" holds last executed command feature

the ilua `_pretty_print` feature has the property that it sets _
to be what was printed, as an interpreter feature so that _
represents the results of the previous expression. However it
appears that this may conflict with some parts of our api, for
instance the _ is used to hold the return value of
wesnoth.textdomain in the lua implementation of [harm_unit].

Note that there is no actual bug that I have observed, I merely
anticipate that this _ feature won't be compatible with our
established gettext conventions.

data/lua/ilua.lua

@@ -9,8 +9,6 @@
 local pretty_print_limit = 20
 local max_depth = 7
 local table_clever = true
--- suppress strict warnings
-_ = true
 
 -- imported global functions
 local sub = string.sub
@@ -102,7 +100,6 @@ function ilua._pretty_print(...)
     for i,val in ipairs(arg) do
         print(ilua.val2str(val))
     end
-    _G['_'] = arg[1]
 end
 
 --