test/eal: set memory channel config only in dedicated test

test/eal: set memory channel config only in dedicated test

The -n option is an optimisation configuration option that defaults to 0.
Such a default value makes the mempool library distributes objects as if
there was 4 memory channels, so -n 4 is the same as the default behavior.

This parameter was mandatory a long time ago, but has been optional for
a while. We check that setting this value works fine in its own test.
Remove it everywhere else.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Aaron Conole <aconole@redhat.com>