Round 4 of remove unused functions from src/libudev/{hashmap,log,set,util}.c

We removed the following functions:

  hashmap_merge
  path_is_read_only_fs
  set_free_free
  set_free_freep
  readlink_malloc
  hexchar
  unhexchar
  octchar
  unoctchar
  format_timespan

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>