param-parse: allow duplicate map keys

param-parse: allow duplicate map keys

It is a bit odd that this is accepted (the second value overwrites the
first):

  -p a=3 -p a=4

but this isn't:

  -p a=3,a=4

Remove the check that prevents duplicate keys from being defined in a
map.

Change-Id: Id39e86bb48ac52c6714db3128b7d767f5347de64
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2423