cli: Remove support for --key and --value arguments

cli: Remove support for --key and --value arguments

There are currently two ways of passing parameters to the current
component on the Babeltrace command line:

  --params=KEY=VALUE
  --key KEY --value VALUE

The method with --params is more versatile, as VALUE can have different
types (string, number, bool).  The value in the --key/--value method is
always converted to a string.

This patch removes the less-versatile --key/--value way, in order to
keep only --params.

I have remove or updated references to --key/--value in man pages.  The
only remaining mention of --key/--value that I know of is in
babeltrace-log.c, which I was told is going to disappear anyway, so I
have left it.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>