Dockerfile: remove GOMETALINTER_OPTS

[thaJeztah]

This ENV was added to the Dockerfile in b96093f (#34759),
when the repository used per-architecture Dockerfiles, and some architectures needed
a different configuration.

Now that we use a multi-arch Dockerfile, and CI uses a Jenkinsfile, we can remove
this ENV from the Dockerfile, and set it in CI instead if needed.

Also updated the wording and fixed linting issues in hack/validate/gometalinter

Note that this changes the --deadline to use the default (10m), which probably isn't a big issue (note that on the Go 1.13 update PR, we actually run into the 2m deadline #39549, which is when I found that we override the default 10m)

[thaJeztah]

ping @kolyshkin @vdemeester PTAL

[vdemeester]

LGTM

[thaJeztah]

Failures on RS1; this one is not (yet) marked as flaky, but was mentioned in #39747 (comment) to be timing out on RS1

Previously it was disabled on Windows (known to be failing); #34340

 --- FAIL: Test/DockerSuite/TestAPIImagesSaveAndLoad (453.65s)
     cli.go:45: assertion failed: 
         Command:  d:\CI-1\CI-a464a3d51\binary\docker.exe inspect --format {{.Id}} sha256:2b2bba38d3ebd66ee344f7fe0e024d00df1b7979b281acddb6130a5e1b86e097
         ExitCode: 1
         Error:    exit status 1
         Stdout:   
         
         Stderr:   Error: No such object: sha256:2b2bba38d3ebd66ee344f7fe0e024d00df1b7979b281acddb6130a5e1b86e097
         
         
         Failures:
         ExitCode was 1 expected 0
         Expected no error

And a known flaky; tracked through issue #38521, and for which there's a WIP PR (that stalled unfortunately); #38595

     --- FAIL: Test/DockerSuite/TestStartReturnCorrectExitCode (8.35s)
         docker_cli_start_test.go:198: assertion failed: 125 (exitCode int) != 12 (int): out: time="2019-09-11T08:32:48Z" level=error msg="error waiting for container: EOF" 

Failure on RS5 was a Jenkins issue https://ci.docker.com/public/job/moby/job/PR-39900/1/execution/node/72/log/

 azwin-2-5f0fe0 was marked offline: Connection was broken: java.util.concurrent.TimeoutException: Ping started at 1568185244721 hasn't completed by 1568185484722
 	at hudson.remoting.PingThread.ping(PingThread.java:134)
 	at hudson.remoting.PingThread.run(PingThread.java:90)
``

[thaJeztah]

Everything else is green; I'm merging this one, because the changes in this PR do not affect anything that's running on Windows, so those failures are definitely unrelated