Remove group column in some summary output tables

[envest]

Files such as results/*train_3_models_summary_table.tsv which are generated using dplyr::group_by() have a column called .groups with values equal to "drop". This looks like either a typo (need to check this) or a package version issue related to now using R3.6.3 after developing in a later version of R. This does not affect performance, it's just an extra unnecessary column of output.