Get rid of message related to reading in a subset of variables

[dtburk]

We shouldn't see this message when we specify a subset of variables with the vars argument to read_ipums_micro():

Note: Using an external vector in selections is ambiguous.
ℹ Use `all_of(vars_of_interest)` instead of `vars_of_interest` to silence this message.
ℹ See <https://tidyselect.r-lib.org/reference/faq-external-vector.html>.

I got this message with ipumsr version 0.4.5 and tidyselect version 1.1.0.