mu4e: Do not defvar abbrev tables

I'm not quite sure why it was done in the first place, and it seems to have
unwanted side-effects. So let's turn it off and see if anything breaks.

Fixes: #2146.

mu4e/mu4e-headers.el

@@ -1083,9 +1083,6 @@ after the end of the search results."
             'field field) " ")))
       mu4e-headers-fields))))
 
-
-(defvar mu4e-headers-mode-abbrev-table nil)
-
 (defun mu4e~headers-maybe-auto-update ()
   "Update the current headers buffer after indexing has brought
 some changes, `mu4e-headers-auto-update' is non-nil and there is

mu4e/mu4e-main.el

@@ -111,10 +111,8 @@ no unread messages.")
     (define-key map "N" 'mu4e-news)
     (define-key map "H" 'mu4e-display-manual)
     map)
-
   "Keymap for the *mu4e-main* buffer.")
 
-(defvar mu4e-main-mode-abbrev-table nil)
 (define-derived-mode mu4e-main-mode special-mode "mu4e:main"
   "Major mode for the mu4e main screen.
 \\{mu4e-main-mode-map}."

mu4e/mu4e-view.el

@@ -960,8 +960,6 @@ This is useful for advising some Gnus-functionality that does not work in mu4e."
   :type 'hook
   :group 'mu4e-view)
 
-(defvar mu4e-view-mode-abbrev-table nil)
-
 ;;  "Define the major-mode for the mu4e-view."
 (define-derived-mode mu4e-view-mode gnus-article-mode "mu4e:view"
   "Major mode for viewing an e-mail message in mu4e.