remove problems attribute from tree_frogs example data

[simonpcouch]

🥲

library(stacks)

format(object.size(tree_frogs), "Kb")
#> [1] "243.7 Kb"

new_tree_frogs <- tree_frogs
attributes(new_tree_frogs)$problems <- NULL

format(object.size(new_tree_frogs), "Kb")
#> [1] "63.6 Kb"

^{Created on 2022-08-25 by the reprex package (v2.0.1)}

Closes #147.

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