Align mobPool enum with other enum naming

[claywar]

Removes plural from filename and enum definition from xi.mobPools to xi.mobPool

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What does this pull request do?

Closes #8564

Other enum definitions use singular case, while mob pools were defined as xi.mobPools.*. Renames the enum and filename to xi.mobPool and mob_pool.lua

Steps to test these changes

No impact to gameplay