rust-dirstatemap: remove unused `_refresh_entry` implementation
This was only used in the newer APIs, all of which have been rewritten in Rust
Differential Revision: https://phab.mercurial-scm.org/D12511
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif