language: remove Currency type.

Users should use Currency type defined in currency package instead.

Also removed Currency from Coverage type. Note that Currency coverage
had not been implemented yet, so we're unlikely to break anyone here.

Change-Id: I094d3bbcec3a05481d627e497daf428191ad2eea
Reviewed-on: https://go-review.googlesource.com/16868
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
11 files changed