Summary:
Before this PR, extra deps passed in from cmd line to buckifier will be parsed
and used to populate a dict. Using this dict and printing to TARGETS file will
lead to printing u'', disallowed by build tools. This PR removes the u''.

Test Plan (local dev server):
```
python buckifier/buckify_rocksdb.py  '{"fake": {"extra_deps": [":test_dep", "//fake/module:mock1"], "extra_compiler_flags": ["-Os", "-DROCKSDB_LITE"]}}'
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6841

Reviewed By: siying

Differential Revision: D21538155

Pulled By: riversand963

fbshipit-source-id: 09403668a4aa1a15bad7dac229c2bc8ce8ee1349