smp_svr/CMakeLists.txt: remove FROM_WHO variable

This variable is unnecessary.  It was a leftover from the original
CMakeLists.txt file used as a template.
diff --git a/samples/smp_svr/zephyr/CMakeLists.txt b/samples/smp_svr/zephyr/CMakeLists.txt
index 7765cb4..612b436 100644
--- a/samples/smp_svr/zephyr/CMakeLists.txt
+++ b/samples/smp_svr/zephyr/CMakeLists.txt
@@ -19,11 +19,6 @@
 
 assert_exists(DTC_OVERLAY_FILE)
 
-# This string ends up getting printed in the device console
-if (NOT DEFINED FROM_WHO)
-  set(FROM_WHO Zephyr)
-endif()
-
 target_sources(app PRIVATE
     src/main.c
 )