This was originally added in
https://github.com/rails/rails/commit/8d26f875f70beef3062deb5ded9c5ffcd473d5ac,
and then updated to something closer to it's current form in
https://github.com/rails/rails/pull/23140

The purpose was to alter the regexp for symbols surrounded by literals
(for path parameters within path segments like "/foo-:bar" or
"/:foo-bar") so that `default_regexp?` would return `false` for those
symbols. `default_regexp?` was then used to partition the routes (see
https://github.com/rails/rails/blob/6ab985da28fd4e2b5c93b50ed4952f43459f8bce/actionpack/lib/action_dispatch/journey/routes.rb#L44).

But after https://github.com/rails/rails/pull/41024, we're no longer
partitioning routes using `default_regexp?` (in fact we're no longer
using the method at all, so I've removed it). So I think it may now be
possible to remove this.

Prior to https://github.com/rails/rails/pull/41024, removing this code
would have broken the tests added in
https://github.com/rails/rails/pull/23140, but those tests now pass
without it. I also tried this patch with the GitHub monolith and didn't
get any test failures.