refactor: remove dotenv as direct dependency

[MasterOdin]

dotenv was used back when ibm-cloud-sdk-core was part of this package, but since it was removed, dotenv is no longer directly used in any of the code except under some of the examples. It does remain in the tree though, so no change to package-lock.json:

ibm-watson@5.0.0-rc1 /Users/mpeveler/work/github/node-sdk
├─┬ ibm-cloud-sdk-core@1.0.0-rc1
│ └── dotenv@6.2.0
└─┬ semantic-release@15.13.24
  └─┬ @semantic-release/npm@5.1.15
    └─┬ npm@6.11.3
      └─┬ libnpx@10.2.0
        └── dotenv@5.0.1

Checklist

• npm test passes (tip: npm run autofix can correct most style issues)

[codecov-io]

Codecov Report

Merging #958 into release-candidate-v5 will not change coverage.
The diff coverage is n/a.

@@                  Coverage Diff                  @@
##           release-candidate-v5     #958   +/-   ##
=====================================================
  Coverage                 64.28%   64.28%           
=====================================================
  Files                         2        2           
  Lines                        14       14           
  Branches                      3        3           
=====================================================
  Hits                          9        9           
  Misses                        4        4           
  Partials                      1        1

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[germanattanasio]

👍

[dpopp07]

Looks good, thanks! 👍