refactor: remove dead make command

LibFDT sources are built directly and not via the `MAKE_LIB` utility
which isn't actually defined in TFTF.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: Ieed18abeade617e741743187fcffa65481b73ed1
diff --git a/lib/libfdt/libfdt.mk b/lib/libfdt/libfdt.mk
index 959d7e2..c3f628d 100644
--- a/lib/libfdt/libfdt.mk
+++ b/lib/libfdt/libfdt.mk
@@ -15,5 +15,3 @@
 			fdt_wip.c)			\
 
 INCLUDES	+=	-Iinclude/lib/libfdt
-
-$(eval $(call MAKE_LIB,fdt))