fs/filesystem: make trait bounds more concise

[00xc]

Fix the following clippy warning that comes up with the nightly toolchain:

warning: this bound is already specified as the supertrait of `DoubleEndedIterator`
   --> src/fs/filesystem.rs:260:47
    |
260 | fn split_path_allow_empty(path: &str) -> impl Iterator<Item = &str> + DoubleEndedIterator {
    |                                               ^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implied_bounds_in_impls
    = note: `#[warn(clippy::implied_bounds_in_impls)]` on by default
help: try removing this bound
    |
260 - fn split_path_allow_empty(path: &str) -> impl Iterator<Item = &str> + DoubleEndedIterator {
260 + fn split_path_allow_empty(path: &str) -> impl DoubleEndedIterator<Item = &str> {