cmd_time: do not show ticks

The command "time" shows the execution time of the command given
to the argument, like this:

    time: 45.293 seconds, 45293 ticks

Since we adopted CONFIG_SYS_HZ = 1000 for all boards,
we always have a simple formula: "1 tick = 0.0001 second".

Showing ticks looks almost redundant.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>