gdb/riscv: Remove use of pseudo registers

gdb/riscv: Remove use of pseudo registers

The code making use of pseudo registers was initially intended to
support running 32-bit ABI files on 64-bit riscv targets.  However, the
implementation was incomplete, and broken.

For now I've removed all reference to pseudo registers from the riscv
target, we've not lost any functionality, and this cleans up failures in
the selftests.

Once the riscv target has matured a little we'll probably end up
bringing back some of the use of pseudo registers in order to better
support running 32-bit executables on a 64-bit target.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_pseudo_register_read): Delete.
	(riscv_pseudo_register_write): Delete.
	(riscv_gdbarch_init): Remove all use of pseudo registers.