util: add removedirs as platform depending function
According to fa901423ac23 introducing "windows._removedirs()":
If a hg repository including working directory is a reparse point
(directory symlinked or a junction point), then using
os.removedirs will remove the reparse point erroneously.
"windows._removedirs()" should be used instead of "os.removedirs()" on
Windows.
This patch adds "removedirs" as platform depending function to replace
"os.removedirs()" invocations for portability and safety