gdb/riscv: Use default gdbarch methods where possible

Make use of the default gdbarch methods for gdbarch_dummy_id,
gdbarch_unwind_pc, and gdbarch_unwind_sp where possible.

This change has been tested with no regressions.

gdb/ChangeLog:

	* gdb/riscv-tdep.c (riscv_dummy_id): Delete.
	(riscv_unwind_pc): Delete.
	(riscv_unwind_sp): Delete.
	(riscv_gdbarch_init): Don't register deleted functions with
	gdbarch.