validation: fix deadlock in XrDestroyInstance
validation: fix deadlock in XrDestroyInstance

GenValidUsageNextXrDestroyInstance locks the non-recursive
g_instance_dispatch_mutex and then calls GenValidUsageCleanUpMaps under the
lock, which calls EraseAllInstanceTableMapElements, which tries to lock
g_instance_dispatch_mutex again, causing a deadlock.
Since EraseAllInstanceTableMapElements is only called from
GenValidUsageNextXrDestroyInstance, g_instance_dispatch_mutex is always
guaranteed to be locked, so drop the local std::unique_lock.

KhronosGroup/OpenXR-SDK,KhronosGroup/OpenXR-SDK,KhronosGroup/OpenXR-SDK