perf data: Correct -h output

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 verbose
        --all             Convert all events
        --to-ctf ...      Convert to CTF format
        --to-json ...     Convert to JSON format
        --tod             Convert time to wall clock time

Signed-off-by: Joshua Martinez <joshuamart@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210824205829.52822-1-irogers@google.com


Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>