[gdb, hurd] Repair build after "Use thread_info and inferior pointers more throughout"

[gdb, hurd] Repair build after "Use thread_info and inferior pointers more throughout"

..., that is commit 00431a78b28f913a9d5c912c49680e39cfd20847 causing:

    [...]/gdb/gnu-nat.c: In member function 'virtual void gnu_nat_target::detach(inferior*, int)':
    [...]/gdb/gnu-nat.c:2284:23: error: invalid conversion from 'int' to 'inferior*' [-fpermissive]
       detach_inferior (pid);
                           ^
    In file included from [...]/gdb/gnu-nat.c:61:0:
    [...]/gdb/inferior.h:523:13: note:   initializing argument 1 of 'void detach_inferior(inferior*)'
     extern void detach_inferior (inferior *inf);
                 ^~~~~~~~~~~~~~~

Fixed by inlining the removed code.

	gdb/
	* gnu-nat.c (gnu_nat_target::detach): Instead of
	'detach_inferior (pid)' call
	'detach_inferior (find_inferior_pid (pid))'.