gdbserver: remove pid_of(thread)

This function doesn't seem so useful, use `thread_info::id::pid`
directly instead.

Change-Id: I7450c4223e5b0bf66788eeb5b070ab6f5287f798
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
6 files changed