Remove `Year` column in `strata_df`

[b-reyes]

Currently, the DataFrame strata_df has the column name year. However, there is no need for this column and it should be removed. To address this issue the following actions should be completed:

• Ensure that removal of the year column is accounted for throughout the codebase
• Check that the file Stratification/US&CAN strata 2019_final.xlsx does not contain data from other years

[b-reyes]

PR #47 addressed this issue, closing.