Drop merging logic for old signatures mode
Drop merging logic for old signatures mode

Summary:
After D27305973, new signatures mode is always enabled. This diff removes the
resulting dead code in the merge service.

Reviewed By: panagosg7

Differential Revision: D27306900

fbshipit-source-id: c8f1109b451f84d7875105c4f524d819cdc7813f

mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow