rrd_map: remove unnecessary use of get().is_some()

rrd_map: remove unnecessary use of get().is_some()

Fixes:

warning: unnecessary use of `get(rel_path).is_some()`
   --> proxmox-rrd/src/cache/rrd_map.rs:107:21
    |
107 |         if self.map.get(rel_path).is_some() {
    |                     ^^^^^^^^^^^^^^^^^^^^^^^ help: replace it with: `contains_key(rel_path)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>