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

crypto/ecies/ecies_test.go

@@ -66,22 +66,6 @@ func cmpParams(p1, p2 *ECIESParams) bool {
 		p1.BlockSize == p2.BlockSize
 }
 
-// cmpPublic returns true if the two public keys represent the same pojnt.
-func cmpPublic(pub1, pub2 PublicKey) bool {
-	if pub1.X == nil || pub1.Y == nil {
-		fmt.Println(ErrInvalidPublicKey.Error())
-		return false
-	}
-	if pub2.X == nil || pub2.Y == nil {
-		fmt.Println(ErrInvalidPublicKey.Error())
-		return false
-	}
-	pub1Out := elliptic.Marshal(pub1.Curve, pub1.X, pub1.Y)
-	pub2Out := elliptic.Marshal(pub2.Curve, pub2.X, pub2.Y)
-
-	return bytes.Equal(pub1Out, pub2Out)
-}
-
 // Validate the ECDH component.
 func TestSharedKey(t *testing.T) {
 	prv1, err := GenerateKey(rand.Reader, DefaultCurve, nil)