gdbserver: Remove thread_to_gdb_id
    
    As explained in the previous patch, the gdb_id concept is no longer
    relevant.  The function thread_to_gdb_id is trivial, it returns the
    thread's ptid.  Remove it and replace its usage with ptid_of.
    
    The changes in nto-low.c and lynx-low.c are fairly straightforward, but
    I was not able to build test them.
    
    gdb/gdbserver/ChangeLog:
    
    	* inferiors.h (thread_to_gdb_id): Remove.
    	* inferiors.c (thread_to_gdb_id): Remove.
    	* server.c (handle_qxfer_threads_worker, handle_query): Adjust.
    	* lynx-low.c (lynx_resume, lynx_wait_1, lynx_fetch_registers,
    	lynx_store_registers, lynx_read_memory, lynx_write_memory):
    	Likewise.
    	* nto-low.c (nto_fetch_registers, nto_store_registers,
    	nto_stopped_by_watchpoint, nto_stopped_data_address): Likewise.