tab: remove_tab() directly, not in an idle

remove_tab() is now called outside of a signal handler, so it is safe to
remove the tab directly.

This commit exposed a bug in GtkSourceCompletionContext, which is now
fixed in commit 31dc73a5e6f53d0c19c6a72b2878d4160208a4e5 (in the
gtksourceview repository).