Remove hex_brightening config option

It is not used, and appears to have been unused for a long time.
It was related to mouseover hex brightening, but the brightening
amount is hardcoded. There were associated hex_semi_brightening,
mouseover_image and selected_image keys, but that info is also
hardcoded, and those keys appear to have been removed already.

data/game_config.cfg

@@ -36,9 +36,6 @@
     # zoom factors must be a sorted list of ascending multipliers
     zoom_levels = 0.25, 0.33333333333333333, 0.5, 0.75, 1.0, 1.25, 1.5, 2.0, 3.0, 4.0
 
-    #temporary disable hex brightening
-    hex_brightening=1.25
-
     flag_rgb=flag_green
     unit_rgb=magenta
     red_green_scale="e60000,ff0000,ff4000,ff8000,ffc000,ffff00,c0ff00,80ff00,40ff00,00ff00,00e600"

data/schema/core/config.cfg

@@ -46,7 +46,6 @@
 	{SIMPLE_KEY hp_bar_scaling real}
 	{SIMPLE_KEY xp_bar_scaling real}
 	{SIMPLE_KEY zoom_levels real_list}
-	{SIMPLE_KEY hex_brightening real}
 	{SIMPLE_KEY flag_rgb string}
 	{SIMPLE_KEY unit_rgb string}
 	{SIMPLE_KEY red_green_scale hex_list}

src/game_config.cpp

@@ -78,8 +78,6 @@ std::vector<unsigned int> zoom_levels {36, 72, 144};
 //
 double hp_bar_scaling  = 0.666;
 double xp_bar_scaling  = 0.5;
-// TODO: highdpi - remove hex_brightening? it is and was not actually used
-double hex_brightening = 1.25;
 
 //
 // Misc
@@ -366,7 +364,6 @@ void load_config(const config &v)
 
 	hp_bar_scaling  = v["hp_bar_scaling"].to_double(0.666);
 	xp_bar_scaling  = v["xp_bar_scaling"].to_double(0.5);
-	hex_brightening = v["hex_brightening"].to_double(1.25);
 
 	foot_speed_prefix   = utils::split(v["footprint_prefix"]);
 	foot_teleport_enter = v["footprint_teleport_enter"].str();

src/game_config.hpp

@@ -152,7 +152,6 @@ namespace game_config
 	extern std::string shroud_prefix, fog_prefix;
 
 	extern double hp_bar_scaling, xp_bar_scaling;
-	extern double hex_brightening; // UNUSED
 
 	extern std::string flag_rgb, unit_rgb;
 	extern std::vector<color_t> red_green_scale;