Remove duplicate key to make node happy
Remove duplicate key to make node happy

Auditors: csilvers

Test Plan:
Ran tools/runtests.py testutil.lint_test.Lint_templates.test in webapp/ and
didn't see the following error (which I got before this change).

My npm is v3.7.3.

======================================================================
ERROR: testutil.lint_test.Lint_templates.test
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/chris/khan/webapp/testutil/lint_test.py", line 59, in test
    self.run_lint([dirname])
  File "/Users/chris/khan/webapp/testutil/lint_test.py", line 80, in run_lint
    files_to_lint)
  File "/Users/chris/khan/webapp/tools/runlint.py", line 149, in run_lint
    for (filename, lineno, error) in fn(files_to_lint):
  File "/Users/chris/khan/webapp/jinja2_lint.py", line 183, in lint_javascript_in_html
    errors = run_eslint(lint_inputs)
  File "/Users/chris/khan/webapp/jinja2_lint.py", line 144, in run_eslint
    raise RuntimeError("Unexpected stderr from eslint:\n%s" % stderr)
RuntimeError: Unexpected stderr from eslint:

/Users/chris/khan/webapp/genfiles/node_modules/eslint/lib/config.js:209
                    throw e;
                    ^
YAMLException: Cannot read config file: /Users/chris/khan/devtools/khan-linter/eslintrc.shared
Error: duplicated mapping key at line 133, column 29:
            "babelHelpers": true,
                                ^
Referenced from: /Users/chris/khan/webapp/../devtools/khan-linter/eslintrc.browser
    at generateError (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:161:10)
    at throwError (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:167:9)
    at storeMappingPair (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:304:7)
    at readFlowCollection (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:719:7)
    at composeNode (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:1328:11)
    at readFlowCollection (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:714:7)
    at composeNode (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:1328:11)
    at readBlockMapping (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:1002:16)
    at composeNode (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:1327:12)
    at readDocument (/Users/chris/khan/webapp/genfiles/node_modules/eslint/node_modules/js-yaml/lib/js-yaml/loader.js:1490:3)

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter