meson: Fix handling of 'demos' parameter

Internally the build script evaluated the 'widgetry' parameter instead
which is just a dependency of 'demos'. Evaluate the correct parameter
to show the correct state in the build summary. Also drop the now obsolete
duplicate check in peas-demo's meson script.

meson.build

@@ -206,7 +206,7 @@ if build_gtk_widgetry and not gtk_dep.found()
   build_gtk_widgetry = false
 endif
 
-build_demos = get_option('widgetry')
+build_demos = get_option('demos')
 if build_demos and not build_gtk_widgetry
   build_demos = false
 endif

peas-demo/meson.build

-if get_option('demos')
-
 peas_demo_name = 'peas-demo'
 peas_demo_string = '@0@-@1@'.format(peas_demo_name, api_version)
 peas_demo_plugins_dir = join_paths(get_option('libdir'), 'peas-demo', 'plugins')
@@ -25,5 +23,3 @@ executable(
 )
 
 subdir('plugins')
-
-endif