tests/kernel/tickless: Remove use of CONFIG_TICKLESS_IDLE_THRESHOLD

This kconfig isn't actually exercised in this test, it's just being
used to compute some sleep durations.  Also I want it gone.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
diff --git a/tests/kernel/tickless/tickless_concept/prj.conf b/tests/kernel/tickless/tickless_concept/prj.conf
index 71b24dc..5213c83 100644
--- a/tests/kernel/tickless/tickless_concept/prj.conf
+++ b/tests/kernel/tickless/tickless_concept/prj.conf
@@ -1,5 +1,4 @@
 CONFIG_ZTEST=y
 CONFIG_PM=y
-CONFIG_TICKLESS_IDLE_THRESH=20
 CONFIG_SYS_CLOCK_TICKS_PER_SEC=100
 CONFIG_MP_NUM_CPUS=1
diff --git a/tests/kernel/tickless/tickless_concept/src/main.c b/tests/kernel/tickless/tickless_concept/src/main.c
index 1875569..b1b9b49 100644
--- a/tests/kernel/tickless/tickless_concept/src/main.c
+++ b/tests/kernel/tickless/tickless_concept/src/main.c
@@ -11,21 +11,20 @@
 #define NUM_THREAD 4
 static K_THREAD_STACK_ARRAY_DEFINE(tstack, NUM_THREAD, STACK_SIZE);
 static struct k_thread tdata[NUM_THREAD];
-/*for those not supporting tickless idle*/
-#ifndef CONFIG_TICKLESS_IDLE
-#define CONFIG_TICKLESS_IDLE_THRESH 20
-#endif
+
+#define IDLE_THRESH 20
+
 /*sleep duration tickless*/
-#define SLEEP_TICKLESS	 k_ticks_to_ms_floor64(CONFIG_TICKLESS_IDLE_THRESH)
+#define SLEEP_TICKLESS	 k_ticks_to_ms_floor64(IDLE_THRESH)
 
 /*sleep duration with tick*/
-#define SLEEP_TICKFUL	 k_ticks_to_ms_floor64(CONFIG_TICKLESS_IDLE_THRESH - 1)
+#define SLEEP_TICKFUL	 k_ticks_to_ms_floor64(IDLE_THRESH - 1)
 
 /*slice size is set as half of the sleep duration*/
-#define SLICE_SIZE	 k_ticks_to_ms_floor64(CONFIG_TICKLESS_IDLE_THRESH >> 1)
+#define SLICE_SIZE	 k_ticks_to_ms_floor64(IDLE_THRESH >> 1)
 
 /*maximum slice duration accepted by the test*/
-#define SLICE_SIZE_LIMIT k_ticks_to_ms_floor64((CONFIG_TICKLESS_IDLE_THRESH >> 1) + 1)
+#define SLICE_SIZE_LIMIT k_ticks_to_ms_floor64((IDLE_THRESH >> 1) + 1)
 
 /*align to millisecond boundary*/
 #if defined(CONFIG_ARCH_POSIX)