Remove hosting methods that always return false (dotnet/coreclr#9930)

Remove hosting methods that always return false (dotnet/coreclr#9930)

* Remove CLRIoCompletionHosted as it always returns false.
* Remove CLRSyncHosted as it always returns false.
* Remove CLRMemoryHosted as it always returns false.
* Remove CLRTaskHosted as it always returns false.
* Remove CLRAssemblyHosted, CLRGCHosted,and CLRSecurityHosted.
They are not called.
* Remove IsThreadPoolHosted as it always returns false.
* Remove EnterRuntime and LeaveRuntime, as they do nothing.
* Add back calls to RevertIfImpersonated and GCX_PREEMP.

I accidentally deleted the call to RevertIfImpersonated instead of just
removing an extra parameter.

When I removed the HR_LEAVE_RUNTIME_HOLDER macro from windowsruntime.h,
I not only removed a LeaveRuntimeHolder but also a GCX_PREEMP. So I added
it back. The holder and GCX_PREEMP where only inserted when the
FEATURE_LEAVE_RUNTIME_HOLDER macro was defined. Since it is always defined,
I removed it. Also as I understand it, you would always want to have a
GCX_PREEMP before calling into the Windows API as not to block the GC,
so I'm not sure why you would want to disable it.

Commit migrated from https://github.com/dotnet/coreclr/commit/51e968b013e9b1582035f202e004ed024f747f4f