remove "difficulty_" from game_data and carryover

we already have this data in the classification.
same will happen to random_mode
removed unneeded check in synced_context.cpp

the plan is to remove "random_mode" form game_data
remove "random_mode_" from game_data

we already have this information in the classification.
fixup 'remove difficulcy_'

otherwise we don't see the difficulcy in the loadgame menu and only see
"UNTLB" instead.
Merge pull request #177 from gfgtdf/remove_diffic

remove "difficulty_" and "random_mode_" from game_data and carryover
(we already have them in the classification)