mu4e: use mu4e-config instead of mu4e-meta

All these changes to avoid:

,----
| mu4e/meson.build:92: WARNING: Source item '/home/djcb/Sources/mu/build/mu4e/mu4e-meta.el' cannot be converted to File object, because it is a generated file. This will become a hard error in the future
`----

This is because we want to byte-compile a file we just before generated using
configure_file. This does not seem like a crazy thing, but meson threatens with
breaking the build at some point in the future.

So instead, we decide _not_ to compile this (very boring) file. But, users may
still have an older mu4e-meta.elc lying around, leading to confusion.

So, let's rename that file and we're golden.

mu4e/mu4e-server.el

@@ -25,7 +25,6 @@
 ;;; Code:
 
 (require 'mu4e-helpers)
-(require 'mu4e-meta)
 
 
 ;;; Configuration