src: logging: Makefile: drop undefined LOG_DRIVER
src: logging: Makefile: drop undefined LOG_DRIVER

Commit <0985a9597bb0348d46c0d18dc548a676bf0ad8e2> added unused variable
so remove it.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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