Remove internal ET capability

[mattw-nws]

Key stakeholders have indicated that ET calculation should not be a framework capability, there is a BMI module that serves this purpose, and use of the internal ET calc method is actively discouraged. The requirement to continue to support it for legacy purposes is causing unnecessary zombie code to creep into major architectural features like #497, increasing complexity and reducing maintainability. This is a roadmap issue to discuss removing the internal ET support in a near-future breaking change.

Current behavior

ET calculation is a 1st-class feature provided to models including legacy and BMI modules, including significant pipelining code within BMI formulations.

Expected behavior

ET calc code and the pipelining is removed from the architecture.

Steps to replicate behavior (include URLs)

Screenshots

[mattw-nws]

See also outstanding #352 and #276 , which would be rendered moot if we cut this capability.