Removed unintended tweak to GP

At present the update statement did nothing as no specialists had
YIELD_CULTURE in the SpecialistYields table but it could have caused
future bugs
Feed The World now gives two food from buildings

The general feeling on the forums is that beliefs need to be tweaked and
especially that feed the world needed to be nerfed:
http://forums.civfanatics.com/showthread.php?t=516391

Spreadsheet also updated
Merge pull request #150 from skodkim/stable

Removed unintended tweak to GP