Uninitialized variable "this_id" in frame.c:get_prev_frame_1.

With a simple Ada program where I have 3 functions, one just calling
the next, the backtrace is currently broken when GDB is compiled
at -O2:

   #0  hello.first () at hello.adb:5
   #1  0x0000000100001475 in hello.second () at hello.adb:10
   Backtrace stopped: previous frame inner to this frame (corrupt stack?)

It turns out that a recent patch deleted the assignment of variable
this_id, making it an unitialized variable:

        * frame-unwind.c (default_frame_unwind_stop_reason): Return
        UNWIND_OUTERMOST if the frame's ID is outer_frame_id.
        * frame.c (get_prev_frame_1): Remove outer_frame_id check.

The hunk in question starts with:

-  /* Check that this frame is not the outermost.  If it is, don't try
-     to unwind to the prev frame.  */
-  this_id = get_frame_id (this_frame);
-  if (frame_id_eq (this_id, outer_frame_id))

(the code was removed as redundant - but removing the assignment
was in fact not intentional).

There is no other code in this function that sets the variable.
Instead of re-adding the statement in the lone section where it is
actually used, I inlined it, and then got rid of the variable
altogether.  This way, and until we start needing this frame ID
in another location within that function, we dont' have to worry
about the variable's validity/lifetime.

gdb/ChangeLog:

        * frame.c (get_prev_frame_1): Delete variable "this_id".
        Replace its use by a call to get_frame_id.