riscv: kconfig: Remove RISCV_IRQ_INTC select

The RISCV_IRQ_INTC configuration symbol is undefined, but RISCV selects
it. Quoting Palmer Dabbelt:

	It looks like this slipped through, the symbol has been renamed
	RISCV_INTC.

No RISCV_INTC configuration symbol has been merged either. Just remove
the RISCV_IRQ_INTC select for now.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>