soc: arm: remove explicit selection of already selected symbols

CPU_CORTEX_M_HAS_SYSTICK and CPU_CORTEX_M_HAS_VTOR do not need
to be explicitely selected in SoC/Boards definitions of
platforms implementing the Cortex-M mainline architecture; they
are already selected by ARMV7_M_ARM8_M_MAINLINE Kconfig symbol.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>