libweave: Remove include of chromeos/map_utils.h

It was used only to find value in std::map.
Also replaces EnumToString was used for mapping.

BUG=brillo:1257
TEST='FEATURES=test emerge-gizmo libweave buffet'

Change-Id: I5eb499a9a14c4b6c769cf4925ccfd4d8f3d6e99b