don't remove SLAVEPASS from environment by default in buildbot-worker

[rutsky]

These lines were introduced in 75f9ad7 by @pieterlexis.
I don't think these changes are required anymore.

Worker configuration created with buildbot-worker create ... doesn't
use environment variables to set password.

Worker configuration created with
master/contrib/docker/worker/Dockerfile optionally uses environment
variable to specify password, but also has it's own code that clears
environment.

So I don't see any supported configuration that requires clearing of the
environment variables.

If user created configuration that uses environment variable to set
password, he must clear environment variables.

@pieterlexis please comment if you think I'm missing something.

[codecov-io]

Current coverage is 84.26%

Merging #2132 into master will not affect coverage as of 075d742

@@            master   #2132   diff @@
======================================
  Files          333     333       
  Stmts        32026   32026       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit          26987   26987       
  Partial          0       0       
  Missed        5039    5039       

Review entire Coverage Diff as of 075d742

---

Uncovered Suggestions

1. +0.11% via ...ot/status/builder.py#384...417
2. +0.09% via ...ot/status/builder.py#456...484
3. +0.09% via ...dbot/changes/mail.py#480...507
4. See 7 more...

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[tardyp]

I think I remember it was because the pass appeared in some log.
I think it is better to erase at the same place as it is actually used. (which is the dockerfile?)

[rutsky]

@tardyp I think so too. Environment variables are outputted in each build; and I know only one place where SLAVEPASS is used (and renamed): https://github.com/buildbot/buildbot/blob/master/master/contrib/docker/worker/buildbot.tac#L21

[tardyp]

indeed, git grep confirms it.