project: Remove default warning level

gnome-calendar uses 1 as the project's default warning level.
However, meson already uses this level as the default warning level.

This patch removes the warning level from project's default options.

https://bugzilla.gnome.org/show_bug.cgi?id=790234

meson.build

@@ -3,7 +3,6 @@ project(
                'c',
            version: '3.27.2',
            license: 'GPL3+',
-   default_options: [ 'buildtype=debugoptimized', 'warning_level=1' ],
      meson_version: '>= 0.42.0'
 )