drivers: slip: Remove stale CMakeLists.txt

The slip driver was moved to drivers/net around the same time as the
conversion to CMake,

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>
diff --git a/drivers/slip/CMakeLists.txt b/drivers/slip/CMakeLists.txt
deleted file mode 100644
index e7fc57d..0000000
--- a/drivers/slip/CMakeLists.txt
+++ /dev/null
@@ -1 +0,0 @@
-zephyr_sources_if_kconfig(slip.c)