gdbserver: remove get_thread_process

Remove the `get_thread_process` function, use `thread_info::process`
instead.

In `server.cc`, use `current_process ()` instead of going through the
current thread.

Change-Id: Ifc61d65852e392d154b854a45d45df584ab3922e
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>