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There is currently only 1 'perf data' command, but supporting extra
commands was breaking the help output. Simplify for now so that the help
output is correct.

Before:
$ perf data -h

 Usage: perf data [<common options>] <command> [<options>]

$ perf data
Usage:
        perf data [<common options>] <command> [<options>]

        Available commands:
         convert        - converts data file between formats

After:
$ perf data

 Usage: perf data convert [<options>]

    -f, --force           don't complain, do it
    -i, --input <file>    input file name
    -v, --verbose         be more verbose
        --all             Convert all events
        --to-ctf ...      Convert to CTF format
        --to-json ...     Convert to JSON format
        --tod             Convert time to wall clock time

$ perf data -h

 Usage: perf data convert [<options>]

    -f, --force           don't complain, do it
    -i, --input <file>    input file name
    -v, --verbose         be more verbos...