don't always copy the underlying config on side filters

I checked all codes that construct side filters but i couldn't find a
single occurance where the side filter object lived longer than the
underlying config object. (there were occurances in
terrain_filter/side_filer where i was not sure so i added an extra
make_safe there). Feels a littlte strange since there should've been a
reason why that make_safe call was added in the first place.

src/side_filter.cpp

@@ -124,7 +124,7 @@ bool side_filter::match_internal(const team &t) const
 	if(cfg_.has_child("has_unit")) {
 		const vconfig & ufilt_cfg = cfg_.child("has_unit");
 		if (!ufilter_) {
-			ufilter_.reset(new unit_filter(ufilt_cfg, fc_));
+			ufilter_.reset(new unit_filter(ufilt_cfg.make_safe(), fc_));
 			ufilter_->set_use_flat_tod(flat_);
 		}
 		bool found = false;

src/terrain/filter.cpp

@@ -166,7 +166,7 @@ bool terrain_filter::match_internal(const map_location& loc, const unit* ref_uni
 		if (!u.valid())
 			return false;
 		if (!cache_.ufilter_) {
-			cache_.ufilter_.reset(new unit_filter(cfg_.child("filter"), fc_));
+			cache_.ufilter_.reset(new unit_filter(cfg_.child("filter").make_safe(), fc_));
 			cache_.ufilter_->set_use_flat_tod(flat_);
 		}
 		if (!cache_.ufilter_->matches(*u, loc))

src/units/filter.hpp

@@ -98,7 +98,6 @@ class unit_filter
 		, impl_(cfg_)
 		, max_matches_(-1)
 	{
-		cfg_.make_safe();
 	}
 
 	unit_filter(const unit_filter&) = default;