Revert "[nrf noup] zephyr: Restore default RTC user channel count"

This reverts commit 50e1caa.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

boot/zephyr/prj.conf

@@ -36,4 +36,3 @@ CONFIG_CBPRINTF_NANO=y
 CONFIG_PICOLIBC=y
 ### Disable malloc arena because we don't need it
 CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0
-CONFIG_NRF_RTC_TIMER_USER_CHAN_COUNT=0