cleanup: Remove ports depending on expired lang/gcc6-aux

Keep ports-mgmt/synth and dependencies (including lang/gcc6-aux itself)
for now as synth is the only Ada port still maintained and might be
somewhat high-profile.

Removed ports:
archivers/zip-ada
cad/ghdl
databases/adabase
databases/apq
databases/apq-mysql
databases/apq-odbc
databases/apq-pgsql
devel/ada-util
devel/adabooch
devel/adaid
devel/ahven
devel/alog
devel/aunit
devel/florist-gpl
devel/gprbuild
devel/libspark2012
devel/matreshka
devel/pcsc-ada
devel/pragmarcs
devel/sdl_gnat
dns/ironsides
graphics/generic_image_decoder
lang/adacontrol
lang/asis
lang/gnat_util
lang/lua-ada
math/plplot-ada
misc/excel-writer
net/adasockets
net/anet
security/libadacrypt
textproc/adabrowse
textproc/templates_parser
textproc/words
textproc/xmlada
x11-toolkits/gtkada
x11-toolkits/gtkada3