gdbserver/ipa/x86: remove unneeded declarations

Spotted some declarations in gdbserver/linux-amd64-ipa.cc that are no
longer needed.  These are:

  1. 'init_registers_amd64_linux' - the comment claims this function
  is auto generated, but I don't believe that this is still the case.
  Also the function is not used in this file,

  2. 'tdesc_amd64_linux' - this variable doesn't seem to exist any
  more, I suspect this was renamed to 'tdesc_amd64_linux_no_xml', but
  neither are used in this file, so lets remove the declaration.

The amd64 in-process-agent still builds fine after this commit.

There should be no user visible changes after this commit.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>