kernel: remove reference to legacy_timer.c in build system

We do not have this file anymore, remove it from the cmake files.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/kernel/CMakeLists.txt b/kernel/CMakeLists.txt
index 3a9c533..7dd0f51 100644
--- a/kernel/CMakeLists.txt
+++ b/kernel/CMakeLists.txt
@@ -27,7 +27,6 @@
 target_sources_ifdef(CONFIG_INT_LATENCY_BENCHMARK kernel PRIVATE int_latency_bench.c)
 target_sources_ifdef(CONFIG_STACK_CANARIES        kernel PRIVATE compiler_stack_protect.c)
 target_sources_ifdef(CONFIG_SYS_CLOCK_EXISTS      kernel PRIVATE timer.c)
-target_sources_ifdef(CONFIG_LEGACY_KERNEL         kernel PRIVATE legacy_timer.c)
 target_sources_ifdef(CONFIG_ATOMIC_OPERATIONS_C   kernel PRIVATE atomic_c.c)
 target_sources_ifdef(CONFIG_PTHREAD_IPC           kernel PRIVATE pthread.c)
 target_sources_if_kconfig(                        kernel PRIVATE poll.c)