Remove monitor test

The `monitor` test was originally written to test GFileMonitor with
directories. Over time, `testfilemonitor` acquired units for testing
directories as well, which made the `monitor` test reduntant.

gio/tests/meson.build

@@ -54,7 +54,6 @@ gio_tests = {
   'io-stream' : {},
   'memory-input-stream' : {},
   'memory-output-stream' : {},
-  'monitor' : {},
   'mount-operation' : {},
   'network-address' : {'extra_sources': ['mock-resolver.c']},
   'network-monitor' : {},