riscv: kconfig: Remove assignment to promptless CONFIG_64BIT symbol

64BIT has no prompt. Assignments in configuration files have no effect
on symbols without prompts. A prompt means the symbol is
user-configurable.

64BIT is instead enabled indirectly through being selected by
BOARD_QEMU_RISCV64, which is enabled in the same configuration file.

Detected through some work-in-progress improved error checking.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>