subrepo: use vfs.rmtree instead of shutil.rmtree This patch also removes useless "shutil" import.

vfs: add rmtree This duplicates "onerror()" function from "svnsubrepo.remove()" for equivalence of replacing in subsequent patch. This "onerror()" function for "shutil.rmtree()" was introduced by 92b0d669637f, which avoids failure of removing svn repository on Windows.

subrepo: use vfs.readdir instead of os.listdir to avoid expensive stat calls "kind" information given from "vfs.readdir()" makes expensive stat calls "os.path.isdir()" and "os.path.islink()" useless.