scheduler: merge Reserve and Unreserve plugins

Previously, separate interfaces were defined for Reserve and Unreserve
plugins. However, in nearly all cases, a plugin that allocates a
resource using Reserve will likely want to register itself for Unreserve
as well in order to free the allocated resource at the end of a failed
scheduling/binding cycle. Having separate plugins for Reserve and
Unreserve also adds unnecessary config toil. To that end, this patch
aims to merge the two plugins into a single interface called a
ReservePlugin that requires implementing both the Reserve and Unreserve
methods.

pkg/scheduler/framework/plugins/legacy_registry.go

@@ -273,7 +273,6 @@ func NewLegacyRegistry() *LegacyRegistry {
 			plugins.Filter = appendToPluginSet(plugins.Filter, volumebinding.Name, nil)
 			plugins.Reserve = appendToPluginSet(plugins.Reserve, volumebinding.Name, nil)
 			plugins.PreBind = appendToPluginSet(plugins.PreBind, volumebinding.Name, nil)
-			plugins.Unreserve = appendToPluginSet(plugins.Unreserve, volumebinding.Name, nil)
 			return
 		})
 	registry.registerPredicateConfigProducer(NoDiskConflictPred,