gdb/guile: remove support for Guile < 2.2

gdb/guile: remove support for Guile < 2.2

I propose to remove the code in guile/scm-ports.c supporting Guile <
2.2.  The rationale is:

 - The code within USING_GUILE_BEFORE_2_2 amounts to about half of the
   file, it makes it much more complicated than it would be otherwise.
   I'm trying to investigate PR 29825 [1] and this is getting in the
   way.
 - Guile 2.2, which would now be the baseline for what we require, is
   now 8 years old.  Guile 2.2 is legacy (the current stable branch is
   3.0), but it is still actively packaged [2][3].
 - The Guile support code doesn't receive as much contribution, testing
   and love as the Python support code, for instance.  We don't have
   cycles to spare to support an obsolete version of Guile.

This patch removes the USING_GUILE_BEFORE_2_2 define and all the
portions of code within `#if USING_GUILE_BEFORE_2_2`.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=29825
[2] https://packages.debian.org/sid/guile-2.2-dev
[3] https://archlinux.org/packages/extra/x86_64/guile2.2/

Change-Id: I926e79cde9835567eb3b7e3d22db402c841b79d7
Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>