chain: dont check for null settings value in overwriteRwSetting

- Deleting the settings when the new value is null is redundant
  as we can achieve the same by just calling `deleteRwSetting`.

- This commit updates `overwriteRwSetting` to instead not check
  for null settings and just go ahead and update the settings with
  whatever was passed.

src/node/interfaces.cpp

@@ -830,7 +830,6 @@ class ChainImpl : public Chain
     }
     bool overwriteRwSetting(const std::string& name, common::SettingsValue value, interfaces::SettingsAction action) override
     {
-        if (value.isNull()) return deleteRwSettings(name, action);
         return updateRwSetting(name, [&](common::SettingsValue& settings) {
             settings = std::move(value);
             return action;