remove branch_method for CMIP7

CMIP7 has removed the branch_method from the global attributes, but we rely on this in a number of functions.

I don't think it is problematic to retain it, but we might want to look at how much work it would be to remove.

Start by locating every reference to the branch_method global attribute and inspect the functions that rely on it. Determine whether those dependencies can be removed for CMIP7 without affecting existing behavior, and confirm that the relevant processing paths no longer require the attribute.

Written by the indexing model from the issue text.