TT#44118 Drop (Build-)Depends on libgtk2-gladexml-simple-perl

libgtk2-gladexml-simple-perl ships /usr/share/perl5/Gtk2/GladeXML/Simple.pm
and gpsketcher, though neither of them seem to be in use by ngcp-schema.

Noticed when building against Debian/buster, which no longer
ships libgtk2-gladexml-simple-perl, see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=904551

Change-Id: Ifcd5b53c9bc95d164e73f8b4ff499b696e5fc8df

debian/control

@@ -16,7 +16,6 @@ Build-Depends:
  libfile-path-tiny-perl,
  libfile-sharedir-perl,
  libgetopt-long-descriptive-perl,
- libgtk2-gladexml-simple-perl,
  libhash-storediterator-perl,
  libmodule-build-perl,
  libquantum-superpositions-perl,
@@ -50,7 +49,6 @@ Depends:
  libfile-path-tiny-perl,
  libfile-sharedir-perl,
  libgetopt-long-descriptive-perl,
- libgtk2-gladexml-simple-perl,
  libhash-storediterator-perl,
  libparse-debianchangelog-perl,
  libquantum-superpositions-perl,