map_rpc_server: Simplify makeSignedMapRoot (#1897)
map_rpc_server: Simplify makeSignedMapRoot (#1897)

This change:
- Removes the unused mapID parameter from makeSignedMapRoot.
- Inlines time.Now computation into it, because all the callers do that.
RJPercival/trillian,gdbelvin/trillian,AlCutter/trillian,AlCutter/trillian,google/trillian,gdbelvin/trillian,google/trillian,RJPercival/trillian,gdbelvin/trillian