RISC-V: Remove ptrace support for vectors

We've found two bugs here: NT_RISCV_VECTOR steps on NT_RISCV_CSR (which
is only for embedded), and we don't have vlenb in the core dumps.  Given
that we've have a pair of bugs croup up as part of the GDB review we've
probably got other issues, so let's just cut this for 6.5 and get it
right.

Fixes: 0c59922c

 ("riscv: Add ptrace vector support")
Reviewed-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20230816155450.26200-2-andy.chiu@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>