Remove ESM15 case as inconsistent with current input of xkpsorb (#283)

[rml599gh]

CABLE

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Description

Removed two ifdef ESM15 cases related to the use of xkpsorb. The ESM15 code option is inconsistent with
how xkpsorb is input in the CABLE3 code.

Fixes #283

Type of change

• [X ] Bug fix

Checklist

• The new content is accessible and located in the appropriate section.
• I have checked that links are valid and point to the intended content.
• I have checked my code/text and corrected any misspellings

Please add a reviewer when ready for review.

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📚 Documentation preview 📚: https://cable--375.org.readthedocs.build/en/375/

[rml599gh]

I've found it quite difficult to document a clean but short test, given this only applies to ESM1.5 with CABLE3 and online tests diverge due to internal variability once any difference is made. Tests I have done suggest any changes are very small. The code indicates it should be the additional phosphorus pools that are impacted (labile, sorbed and occluded).
This figure shows the difference in globally averaged occluded phosphorus pool by pft (y axis) with no tile fraction weighting after running for 1 month. The difference is from the original ESM1.5 case. Black and blue are cases with the fix described in this pull request (one uses Leuning, the other Medlyn). Green and red are cases without the fix (one uses rootbeta, the other froot). Note that all differences are tiny compared to the pool size.

The differences do not separate as clearly for the labile and sorbed phosphorus pools but all differences remain tiny.