package/exim: remove unused Local-Makefile

package/exim: remove unused Local-Makefile

When the exim package was added, it included a template for the
build-time configuration file Local/Makefile in the package directory.
In commit 8956779d5d2 (exim: use a more standard build-time
configuration), this template file was replaced with the EDITME
template file included in the exim sources. However, that commit did
not remove Buildroot's Local-Makefile.

Since this file is no longer used, remove it.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>