improve the std.fs.deleteTree API to not require an allocator

[andrewrk]

Extracted from a TODO comment:

zig/std/fs.zig

Lines 293 to 294 in bdfb314

	/// TODO determine if we can remove the allocator requirement
	pub fn deleteTree(allocator: *Allocator, full_path: []const u8) DeleteTreeError!void {

Once #2885 is solved, it should be possible to implement deleteTree without an allocator.

Currently, this function is recursive, which would require an allocator once we have #1006. However, I believe it can be implemented non-recursively, and that it would be a reasonable implementation. It would mean making a few more syscalls, but I think that's generally a nice tradeoff to avoid an entire failure mode.