Documentation/technical/api-hashmap: remove source highlighting

The highlighting was pretty, but unfortunately, the failure mode
when source-highlight is not installed was that the entire code
block disappears.

See https://bugs.debian.org/745591,
    https://bugs.launchpad.net/bugs/1316810.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>