Remove bs3compat's dependence on utility classes

It appears utility classes are responsible for about 1/2 of Bootstrap CSS:

bs_size <- function(x) {
  bs <- bs_theme_dependencies(x)[[2]]
  f <- file.path(bs$src, bs$stylesheet)
  file.info(f)["size"]
}

bs_size(bs_theme())
#> bootstrap.min.css  227346

bs_theme() %>% bs_remove("_utilities") %>% bs_size()
#>   Error: The target selector was not found.
#>         Use "@extend .text-muted !optional" to avoid this error.
#>         on line 2 of inst/bs3compat/_help_text.scss
#>  >>   @extend .form-text; @extend .text-muted;
#>   ------------------------------^

bs_theme() %>% bs_remove(c("bs3compat", "_utilities")) %>% bs_size()
#> bootstrap.min.css       112914

More generally, seems we should favor Sass variables over @extend in bs3compat so it's easier to bs_remove() rules.

Start by reproducing the bs_remove() examples and reading inst/bs3compat/_help_text.scss, where the reported @extend failure occurs. Inspect the other bs3compat Sass files for similar @extend usage and verify that removing utility rules succeeds without errors and produces the smaller stylesheet described in the issue.