Remove the browser plugin

The browser plugin is crashy and broken; there are dozens of bugs filed
against it on Bugzilla and nobody is looking at them. Chrome and Firefox
have both dropped support for NPAPI plugins. Epiphany still has support,
but it's hidden behind a gsetting and all the UI to enable it has been
removed, so very few users would be able to figure out how to enable.
I've even previously considered blacklisting this plugin in the past due
to all the crashes.

Since this plugin has not actually worked in any browsers for a long
time now, time to delete it.

https://bugzilla.gnome.org/show_bug.cgi?id=766776

meson.build

@@ -54,13 +54,10 @@ convertdir = join_paths(datadir, 'GConf', 'gsettings')
 desktopdir = join_paths(datadir, 'applications')
 ifacedir = join_paths(datadir, 'dbus-1', 'interfaces')
 localedir = join_paths(datadir, 'locale')
-mozplugindir = join_paths(libdir, 'mozilla', 'plugins')
 portaldir = join_paths(datadir, 'xdg-desktop-portal', 'portals')
 schemadir = join_paths(datadir, 'glib-2.0', 'schemas')
 servicedir = join_paths(datadir, 'dbus-1', 'services')
 
-plugindir = get_variable('BROWSER_PLUGIN_DIR', mozplugindir)
-
 # XXX: Once https://github.com/systemd/systemd/issues/9595 is fixed and we can
 # depend on this version, replace with something like:
 # systemduserunitdir = systemd_dep.get_pkgconfig_variable('systemduserunitdir',
@@ -134,12 +131,6 @@ else
   have_systemd = false
 endif
 
-if get_option('browser_plugin')
-  json_glib_dep = dependency('json-glib-1.0', version: json_glib_req)
-
-  subdir('browser-plugin')
-endif
-
 if get_option('man')
   xsltproc = find_program('xsltproc')