Remove some unneeded imports and a variable

These were flagged by `cargo clippy`:

    warning: returning the result of a `let` binding from a block
    warning: this import is redundant

src/iface/ethernet.rs

@@ -1676,14 +1676,13 @@ impl<'b, 'c, 'e> InterfaceInner<'b, 'c, 'e> {
     fn igmp_leave_packet<'any>(&self, group_addr: Ipv4Address) -> Option<IpPacket<'any>> {
         self.ipv4_address().map(|iface_addr| {
             let igmp_repr = IgmpRepr::LeaveGroup { group_addr };
-            let pkt = IpPacket::Igmp((Ipv4Repr {
+            IpPacket::Igmp((Ipv4Repr {
                 src_addr:    iface_addr,
                 dst_addr:    Ipv4Address::MULTICAST_ALL_ROUTERS,
                 protocol:    IpProtocol::Igmp,
                 payload_len: igmp_repr.buffer_len(),
                 hop_limit:   1,
-            }, igmp_repr));
-            pkt
+            }, igmp_repr))
         })
     }
 }

src/phy/sys/linux.rs

@@ -1,5 +1,3 @@
-use libc;
-
 #[cfg(any(feature = "phy-raw_socket",
           feature = "phy-tap_interface"))]
 pub const SIOCGIFMTU:   libc::c_ulong = 0x8921;
@@ -14,4 +12,3 @@ pub const TUNSETIFF:    libc::c_ulong = 0x400454CA;
 pub const IFF_TAP:      libc::c_int   = 0x0002;
 #[cfg(feature = "phy-tap_interface")]
 pub const IFF_NO_PI:    libc::c_int   = 0x1000;
-

src/phy/sys/mod.rs

@@ -1,6 +1,5 @@
 #![allow(unsafe_code)]
 
-use libc;
 use std::{mem, ptr, io};
 use std::os::unix::io::RawFd;
 use crate::time::Duration;

src/phy/sys/raw_socket.rs

@@ -1,6 +1,5 @@
 use std::{mem, io};
 use std::os::unix::io::{RawFd, AsRawFd};
-use libc;
 use super::*;
 use crate::wire::EthernetFrame;
 

src/phy/sys/tap_interface.rs

@@ -1,6 +1,5 @@
 use std::io;
 use std::os::unix::io::{RawFd, AsRawFd};
-use libc;
 use super::*;
 use crate::wire::EthernetFrame;