Most of the targets in the Makefile have not been used in either CI or
the normal development workflow for some time. Removing them clarifies
that the expected way to build Terraform locally is simple: go install.

Remaining targets:

- fmtcheck, generate: these are used in CI to verify that the code is
  correctly formatted and that generate has been run appropriately
- protobuf: referenced in CONTRIBUTING.md as the simplest way to build
  the proto files
- website, website-test: used to compile and test the local website in
  isolation from the terraform-website repo