Remove some extraneous changes in readline/terminal.c

Remove some extraneous changes in readline/terminal.c

This patch removes some unnecessary differences between the official
version of readline, and GDB's copy of it.

readline/ChangeLog.gdb:

        * terminal.c: Remove duplicate includes of windows.h and
        wincon.h.
        (_rl_get_screen_size): Remove redundant code for MinGW getting
        the console size from the Windows API.