rados tool: remove -t param option for target pool

rados tool: remove -t param option for target pool

Bug #2772. This fixes an issue that was introduced when we
added the 'rados cp' command. The -t param was already used
for rados bench. With this change the only way to specify
a target pool is using --target-pool.
Though this problem is post argonaut, the 'rados cp' command
has been backported, so we need this fix there too.

Backport: argonaut

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>