Revert "[libc++] Fix `regex_search` to match `$` alone with `match_default` flag" (#78349)

The behavior of `std::regex_search` for patterns anchored both to the
start and to the end of the input went wrong after merging #77256 .
Patterns like `"^b*$"` started matching the strings such as `"a"`, which
is not expected.

Reverts the PR: #77256