[geary] Remove composer pref text in the online manual, it no longer exists.

[Michael Gratton <mjog src gnome org>]

commit e6ecefd8276d60d5bcd916af7fbaeb09f0e170b8
Author: Michael James Gratton <mike vee net>
Date:   Thu Jan 11 02:15:37 2018 +1100

    Remove composer pref text in the online manual, it no longer exists.

 help/C/preferences.page |   13 +------------
 1 files changed, 1 insertions(+), 12 deletions(-)
---
diff --git a/help/C/preferences.page b/help/C/preferences.page
index db050e3..a518fd6 100644
--- a/help/C/preferences.page
+++ b/help/C/preferences.page
@@ -35,18 +35,7 @@
        </item>
        </terms>
        </section>
-       
-       <section id="composer">
-       <title>Composer</title>
-       <terms>
-       <item>
-       <title><gui>Enable spell checking</gui></title>
-       <p>When set, Geary automatically spell checks a message as you write it, underlying
-       each misspelled word in red.</p>
-       </item>
-       </terms>
-       </section>
-       
+
        <section id="notifications">
        <title>Notifications</title>
        <terms>