SERVER-127589 Introduces extensionsConfigPath, which allows users to specify the path at which extension configuration files are expected to be found. In order to not break Atlas immediately before they have a chance to implement the required changes to provide the path to the server, we provided a fallback. If no value is provided in the config parameters at startup time, the server defaults to /etc/mongo/extensions.

As part of this ticket, remove the fallback and tassert we have a valid value present when loading extensions.