tab: remove comments explaining standard GObject code

The comments clutter the code and make them less readable. For someone
unfamiliar with GObject the best is to first learn GObject.
tab: remove unused #include's
tab: simplify install_auto_save_timeout_if_needed()

The return value was never used.
tab: remove install_auto_save_timeout()

Use only install_auto_save_timeout_if_needed(). It simplifies the code.
tab: simplify auto save timeout removing
tab: update_auto_save_timeout() in gedit_tab_set_state()

The function adds or removes the timeout according to the state and
other properties. The function is called in gedit_tab_set_state(), so
other GeditTab features don't need to explicitely remove/add the auto
save timeout a bit everywhere in the code...