jiri: Remove CurrentManifest.
jiri: Remove CurrentManifest.

This CL removes CurrentManifest, and the methods to write and read it.

The one caller of CurrentManifest() has been changed to use the latest
update snapshot.

MultiPart: 2/2
Change-Id: I4ed89b3912e6f7ce542c306b2f04351a5c2d8385

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools