execline: remove --shebangdir configure option

The host build passes the --shebangdir configure option, the target
build doesn't.  With the removal of $(HOST_DIR)/usr, it is not clear
if this should be /bin or /usr/bin or $(HOST_DIR)/bin. Looking at the
source code, it turns out that this variable is not used at all, and
/usr/bin doesn't appear anywhere in the installed files.

Since it is not clear what this option should be set to, and it
anyway doesn't do anything, remove it entirely.

Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>