gdbserver: remove declaration of init_registers_arm_with_neon

The last use of init_registers_arm_with_neon was removed in this
commit:

  commit 7cc17433020a62935e4d91053251fe900d83c7f0
  Date:   Fri Jul 19 15:04:48 2019 +0100

      Arm: Use read_description funcs in gdbserver

But the declaration was left around.  Remove the declaration now.