Remove exclude for test_gc_parameter_init_slots

The test no longer exists.

Author: peterzhu2118

test/.excludes-mmtk/TestGc.rb

@@ -6,7 +6,6 @@
 exclude(:test_gc_config_setting_returns_updated_config_hash, "testing behaviour specific to default GC")
 exclude(:test_gc_internals, "testing behaviour specific to default GC")
 exclude(:test_gc_parameter, "testing behaviour specific to default GC")
-exclude(:test_gc_parameter_init_slots, "testing behaviour specific to default GC")
 exclude(:test_heaps_grow_independently, "testing behaviour specific to default GC")
 exclude(:test_latest_gc_info, "testing behaviour specific to default GC")
 exclude(:test_latest_gc_info_argument, "testing behaviour specific to default GC")