build: use golangci-lint (#20295)

* build: use golangci-lint

This changes build/ci.go to download and run golangci-lint instead
of gometalinter.

* core/state: fix unnecessary conversion

* p2p/simulations: fix lock copying (found by go vet)

* signer/core: fix unnecessary conversions

* crypto/ecies: remove unused function cmpPublic

* core/rawdb: remove unused function print

* core/state: remove unused function xTestFuzzCutter

* core/vm: disable TestWriteExpectedValues in a different way

* core/forkid: remove unused function checksum

* les: remove unused type proofsData

* cmd/utils: remove unused functions prefixedNames, prefixFor

* crypto/bn256: run goimports

* p2p/nat: fix goimports lint issue

* cmd/clef: avoid using unkeyed struct fields

* les: cancel context in testRequest

* rlp: delete unreachable code

* core: gofmt

* internal/build: simplify DownloadFile for Go 1.11 compatibility

* build: remove go test --short flag

* .travis.yml: disable build cache

* whisper/whisperv6: fix ineffectual assignment in TestWhisperIdentityManagement

* .golangci.yml: enable goconst and ineffassign linters

* build: print message when there are no lint issues

* internal/build: refactor download a bit

core/forkid/forkid.go

@@ -186,13 +186,6 @@ func newFilter(config *params.ChainConfig, genesis common.Hash, headfn func() ui
 	}
 }
 
-// checksum calculates the IEEE CRC32 checksum of a block number.
-func checksum(fork uint64) uint32 {
-	var blob [8]byte
-	binary.BigEndian.PutUint64(blob[:], fork)
-	return crc32.ChecksumIEEE(blob[:])
-}
-
 // checksumUpdate calculates the next IEEE CRC32 checksum based on the previous
 // one and a fork block number (equivalent to CRC32(original-blob || fork)).
 func checksumUpdate(hash uint32, fork uint64) uint32 {