Don't complete --help flag when flag parsing disabled (#2061)

Fixes #2060

When a command sets `DisableFlagParsing = true` it requests the
responsibility of doing all the flag parsing. Therefore even the
`--help/-f/--version/-v` flags should not be automatically completed
by Cobra in such a case.

Without this change the `--help/-h/--version/-v` flags can end up being
completed twice for plugins: one time from cobra and one time from the
plugin (which has set `DisableFlagParsing = true`).

Signed-off-by: Marc Khouzam <marc.khouzam@gmail.com>