Remove multiplatform support from Makefile

[piersy]

Problem

We have a lot of targets in our Makefile devoted to building for different platforms. We don't actually need them, I checked with @shapeshed and the only platform we need to support right now is linux x64.

The existence of these extra targets makes updating the Makefile laborious and also precipitates the need for the build system (ci.go), which if not providing support for multi-platform builds only serves to obfuscate our build and test operations (see discussion on #474 ).

Acceptance Criteria

Remove all multi-platform targets from the Makefile
Remove ci.go and the associated build package and all use of the build system.