server: Remove --restart-epoch command-line option because #9888 computes it automatically

Currently, a deployment using hot-restart specifies a `--restart-epoch epoch_number` as a command-line argument to the server. Once #9888 is merged, `server.hot_restart_generation` will be computed automatically, eliminating the need for scripting to bump the epoch manually.

We should probably not remove that command-line option, however, until sometime after #4980 is merged, because there will be a discontinuity in the `server.hot_restart_generation` stat.

Then there probably should be a merger of the two stats, eventually dropping one.