lib: os: remove dead code

If multithreading is disabled, thread_entry() never runs
since we cannot create threads; the non-multithreading case
was simply dead code.

Indicate to code coverage that CODE_UNREACHABLE should be
skipped.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

lib/os/thread_entry.c

@@ -28,18 +28,12 @@ FUNC_NORETURN void z_thread_entry(k_thread_entry_t entry,
 {
 	entry(p1, p2, p3);
 
-#ifdef CONFIG_MULTITHREADING
 	k_thread_abort(k_current_get());
-#else
-	for (;;) {
-		k_cpu_idle();
-	}
-#endif
 
 	/*
 	 * Compiler can't tell that k_thread_abort() won't return and issues a
 	 * warning unless we tell it that control never gets this far.
 	 */
 
-	CODE_UNREACHABLE;
+	CODE_UNREACHABLE; /* LCOV_EXCL_LINE */
 }