core/shed: remove not needed bitarithm include add missing

bitarithm.h is not needed for the interface of shed but may cause conflicts
due to different definitions of SETBIT and CLRBIT

common implementations are: (value, offset) xor (value, mask) bitarithm
implements the later

frac.c and nrf52/usbdev.c use bitarithm.h but where missing the include

sam0/rtt.c defined a bit using mask from bitarithm,
changed that to the soulution used in sam0/rtc.c

core/include/sched.h

@@ -82,7 +82,6 @@
 
 #include <stddef.h>
 #include "kernel_defines.h"
-#include "bitarithm.h"
 #include "kernel_types.h"
 #include "native_sched.h"
 #include "clist.h"

cpu/nrf52/periph/usbdev.c

@@ -31,6 +31,7 @@
 #include "periph/usbdev.h"
 #include "usb.h"
 #include "usb/descriptor.h"
+#include "bitarithm.h"
 
 #define ENABLE_DEBUG (0)
 #include "debug.h"

cpu/sam0_common/periph/rtt.c

@@ -32,7 +32,8 @@
  * effects, but simplifies the code. (This bit is always set on SAML21xxxxA)
  */
 #ifndef RTC_MODE0_CTRLA_COUNTSYNC
-#define RTC_MODE0_CTRLA_COUNTSYNC   BIT15
+#define RTC_MODE0_CTRLA_COUNTSYNC_Pos   15
+#define RTC_MODE0_CTRLA_COUNTSYNC       (0x1ul << RTC_MODE0_CTRLA_COUNTSYNC_Pos)
 #endif
 
 static rtt_cb_t _overflow_cb;

sys/frac/frac.c

@@ -19,6 +19,7 @@
 #include <stdio.h>
 #include "assert.h"
 #include "frac.h"
+#include "bitarithm.h"
 
 #define ENABLE_DEBUG (0)
 #include "debug.h"