[geary] Remove deprecated valac --thread arg

[Michael Gratton <mjog src gnome org>]

commit 4d4ab6000f20ddcb9856b29982a1abf3031cb875
Author: Michael Gratton <mike vee net>
Date:   Fri Mar 15 16:52:47 2019 +1100

    Remove deprecated valac --thread arg

 src/meson.build | 1 -
 1 file changed, 1 deletion(-)
---
diff --git a/src/meson.build b/src/meson.build
index c3fb8c6d..ea5509a9 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -20,7 +20,6 @@ endif
 # Common vala options
 geary_vala_options = [
   '--target-glib=@0@'.format(target_glib),
-  '--thread',
   '--enable-checking',
 ]