remove two not-needed functions

[kmdeck]

Purpose

These functions are not needed because (1) they are extremely simply (just returning something) and (2) we do not have multiple methods and do not have a need to specialize.

To-do

Content

Remove ClimaLand.liquid_precipitation and ClimaLand.snow_precipitation.

Review checklist

I have:

• followed the codebase contribution guide: https://clima.github.io/ClimateMachine.jl/latest/Contributing/
• followed the style guide: https://clima.github.io/ClimateMachine.jl/latest/DevDocs/CodeStyle/
• followed the documentation policy: https://github.com/CliMA/policies/wiki/Documentation-Policy
• checked that this PR does not duplicate an open PR.

In the Content, I have included

• relevant unit tests, and integration tests,
• appropriate docstrings on all functions, structs, and modules, and included relevant documentation.

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• I have read and checked the items on the review checklist.

[akshaysridhar]

LGTM, we can merge it once it's rebased.