gtd-window: remove unused function gtd_window_get_manager

gtd_window_get_manager does not seem to be used
anywhere in the code. It also causes to print
a warning during the build.

Remove this unused function to mute this warning.

GNOME/gnome-todo#93
3 jobs for fix-build-warnings in 11 minutes and 56 seconds (queued for 33 minutes and 5 seconds)
latest
Status Job ID Name Coverage
  Flatpak
passed #97786
flatpak:master

00:11:36

 
  Review
passed #97787
review

00:00:20

manual #97788
allowed to fail manual
stop_review