Remove throw tests from request._pruneKeys.
Remove throw tests from request._pruneKeys.

These errors are not described as expected behavior and are merely a
side effect of implementation.

rcchan/mongo-web-shell,10gen-labs/mongo-web-shell,mongodb-labs/mongo-web-shell,mcomella/mongo-web-shell,ecbtln/mongo-web-shell,10gen-labs/mongo-web-shell,mcomella/mongo-web-shell,10gen-labs/mongo-web-shell,mongodb-labs/mongo-web-shell,xl76/mongo-web-shell,mcomella/mongo-web-shell,lnickers2004/mongo-web-shell,ecbtln/mongo-web-shell,rcchan/mongo-web-shell,FuegoFro/mongo-web-shell,pilliq/mongo-web-shell,mongodb-labs/mongo-web-shell,pilliq/mongo-web-shell,FuegoFro/mongo-web-shell,ecbtln/mongo-web-shell,pilliq/mongo-web-shell,FuegoFro/mongo-web-shell,lnickers2004/mongo-web-shell,rcchan/mongo-web-shell,xl76/mongo-web-shell,lnickers2004/mongo-web-shell,lnickers2004/mongo-web-shell