The MTA has been hiring pricey private lawyers to defend itself against injury claims — but refuses to reveal the full scale of the outsourced multimillion-dollar legal work and total cost to taxpayers.  The agency, when pressed by The Post in response to a Freedom of Information Law request, admitted it paid more than $10 million to six law firms in recent years — but sources with knowledge of the situation claim that figure is just the tip of the fiscal iceberg.  “Millions of dollars are being paid out to outside counsel, and there’s no accountability,” said a lawyer in the MTA division that helps oversee such cases.  A cop investigates an accident involving an MTA bus on the Upper East Side in April.…