Revert "Add [modify_unit]rebuild=no"

This reverts commit 51d6402453.

After discussion with @gfgtdf it was determined that this is not a useful feature after all, since it was in fact the default behaviour for the optimized path.

changelog.md

@@ -54,7 +54,6 @@
    * [effect]apply_to=variation now supports heal_full
    * Support [set/clear_variable] inside [modify_unit/side]
    * Support [variables] in [modify_side], as in [modify_unit]
-   * [modify_unit] now accepts rebuild=no which will avoid a rebuild of the unit if possible (not guaranteed)
  ### Packaging
    * The Wesnoth client now looks for the data/dist file when logging into the multiplayer server.
      This file should contain one of the following values based on where the package is for:

data/lua/wml/modify_unit.lua

@@ -53,7 +53,6 @@ local known_attributes = make_set {
 	"description",
 	"hidden",
 	"unrenamable",
-	"rebuild",
 }
 
 local known_tags = make_set {
@@ -168,10 +167,10 @@ local function simple_modify_unit(cfg)
 			u:transform(cfg.type or u.type, cfg.variation)
 		end
 
-		-- by default always do an advancement here (not only when experience/max_experience/type was modified)
+		-- always do an advancement here (not only when experience/max_experience/type was modified)
 		-- for compatibility with old code.
 		-- Skip for recall list units
-		if u.valid == 'map' and (cfg.experience or cfg.max_experience or cfg.rebuild ~= false) then
+		if u.valid == 'map' then
 			u:advance()
 		end
 	end