flex: remove bogus comment

The comment that explains why we were defining HOST_FLEX_DEPENDENCIES
instead of inheriting from FLEX_DEPENDENCIES no longer makes sense,
since we remove such automatic inheritance of host dependencies from
target dependencies a while ago.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>