don't specify default values for reversible blocks db configs

[spoonincode]

Change Description

Specifying default values for these makes nodeos always print

warn  2021-04-27T14:17:13.669 nodeos.Ma chain_plugin.cpp:875          plugin_initialize    ] reversible-blocks-db-size-mb deprecated and will be removed in future version
warn  2021-04-27T14:17:13.669 nodeos.Ma chain_plugin.cpp:878          plugin_initialize    ] reversible-blocks-db-guard-size-mb deprecated and will be removed in future version

on start up, even if the user didn't set them. Removing the default value means they're only printed if the user has actually set the config, which I think is more in line with the intention?

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