Revert "sd-dhcp-client: use asynchronous_close()"

Revert "sd-dhcp-client: use asynchronous_close()"

This effectively reverts the commit 22fc2420b2a7220addcee33c2fa17ada44d87f9c.

The function `asynchronous_close()` confuses valgrind. Before this commit,
valgrind may report the following:

```
HEAP SUMMARY:
    in use at exit: 384 bytes in 1 blocks
  total heap usage: 4,787 allocs, 4,786 frees, 1,379,191 bytes allocated

384 bytes in 1 blocks are possibly lost in loss record 1 of 1
   at 0x483CAE9: calloc (vg_replace_malloc.c:760)
   by 0x401456A: _dl_allocate_tls (in /usr/lib64/ld-2.31.so)
   by 0x4BD212E: pthread_create@@GLIBC_2.2.5 (in /usr/lib64/libpthread-2.31.so)
   by 0x499B662: asynchronous_job (async.c:47)
   by 0x499B7DC: asynchronous_close (async.c:102)
   by 0x4CFA8B: client_initialize (sd-dhcp-client.c:696)
   by 0x4CFC5E: client_stop (sd-dhcp-client.c:725)
   by 0x4D4589: sd_dhcp_client_stop (sd-dhcp-client.c:2134)
   by 0x493C2F: link_stop_clients (networkd-link.c:620)
   by 0x4126DB: manager_free (networkd-manager.c:867)
   by 0x40D193: manager_freep (networkd-manager.h:97)
   by 0x40DAFC: run (networkd.c:20)

LEAK SUMMARY:
   definitely lost: 0 bytes in 0 blocks
   indirectly lost: 0 bytes in 0 blocks
     possibly lost: 384 bytes in 1 blocks
   still reachable: 0 bytes in 0 blocks
        suppressed: 0 bytes in 0 blocks

For lists of detected and suppressed errors, rerun with: -s
ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
```