shelve: use tryunlink

vfs: add tryunlink method Thoughout hg code, we see a pattern of attempting to remove a file and then catching and ignoring any errors due to a missing file in the calling code. Let's unify this pattern in a single implementation in the vfs layer.

util: use tryunlink in unlinkpath We just introduced a func to attempt a file removal. Start using it.

util: add tryunlink function Throughout mercurial cdoe, there is a common pattern of attempting to remove a file and ignoring ENOENT errors. Let's move this into a common function to allow for cleaner code.

util: unify unlinkpath Previously, there were two slightly different versions of unlinkpath between windows and posix, but these differences were eliminated in previous patches. Now we can unify these two code paths inside of the util module.

posix: use local reference to removedirs We have a local reference to os.removedirs in module scope, but we still used os.removedirs inside functions. This changes util to use the local reference, which will pave the way for combining duplicated code in future patches.