DESPERATE situations brook desperate measures. Living under constant threats from extortionists, which the underworld unfailingly backs up with bullets, Bombays business community has now decided to stop paying taxes. This, businessmen feel, is the only way they can remind the state governmenthowever rudelyof its duty to protect the life and limb of its citizens. The tinge of paranoia is quite natural: this month alone, the city police has stood helpless as 19 killings by the underworld underscored the extortion menace.