gdb: remove manual frame_info reinflation code in backtrace_command_1

gdb: remove manual frame_info reinflation code in backtrace_command_1

With the following patch applied (gdb: use frame_id_p instead of
comparing to null_frame_id in frame_info_ptr::reinflate), I would get:

    $ ./gdb -q -nx --data-directory=data-directory testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame -ex "b breakpt" -ex r -ex "bt full"
    Reading symbols from testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame...
    Breakpoint 1 at 0x1131: file /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c, line 22.
    Starting program: /home/smarchi/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/bt-selected-frame/bt-selected-frame
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

    Breakpoint 1, breakpt () at /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c:22
    22      }
    #0  breakpt () at /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.base/bt-selected-frame.c:22
    No locals.
    /home/smarchi/src/binutils-gdb/gdb/frame-info.c:42: internal-error: reinflate: Assertion `frame_id_p (m_cached_id)' failed.

This is because the code in backtrace_command_1 to manually reinflate
`fi` steps overs frame_info_ptr's toes.

When calling

    fi.prepare_reinflate ();

`fi` gets properly filled with the cached frame id.  But when this
happens:

    fi = frame_find_by_id (frame_id);

`fi` gets replaced by a brand new frame_info_ptr that doesn't have a
cached frame id.  Then this is called without a cached frame id:

    fi.reinflate ();

That doesn't cause any problem currently, since

 - the gdb_assert in the reinflate method doesn't actually do anything
   (the following patch fixes that)
 - `fi.m_ptr` will always be non-nullptr, since we just got it from
   frame_find_by_id, so reinflate will not do anything, it won't try to
   use m_cached_id

Fix that by removing the code to manually re-fetch the frame.  That
should be taken care of by frame_info_ptr::reinflate.

Note that the old code checked if we successfully re-inflated the frame
or not, and if not it did emit a warning.  The equivalent in
frame_info_ptr::reinflate asserts that the frame has been successfully
re-inflated.  It's not clear if / when this can happen, but if it can
happen, we'll need to find a solution to this problem globally
(everywhere a frame_info_ptr can be re-inflated), not just here.  So I
propose to leave it like this, until it does become a problem.

Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Change-Id: I07b783d94e2853e0a2d058fe7deaf04eddf24835