Simplify Makefile

[steliyan]

Fixed

.PHONY was a variable assignment, not PHONY declaration (:= vs :).

• Consistent @ usage - we want verbose output in the CI, no need to silence it (can be simulated with make -s)

Removed

• GO_FILES - golangci-lint takes care of everything
• coverage - we don't really track the coverage. If we decide to integrate it (or visualize it), we can bring it back.
• vendor goal - no vendored dependencies
• generate goal - no go:generate usage in the entire codebase
• clean goal - not needed after removal of the test coverage
• vet goal - already included in the golangci-run config
• goimports goal - already included in the golangci-run config
• count=1 - for CI it doesn't matter, locally we can use the cache

Renamed

• Rename fmtcheck goal to lint
• Rename all goal to ci

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