Delete Thread-related dead code (#450)

Delete Thread-related dead code (#450)

* Delete Thread::m_threadsAtUnsafePlaces

Never used. Always zero.

* Delete Thread::m_pLoadingFile

Never used

* Delete Thread::m_nNestedMarshalingExceptions

Never used

* Delete Thread::m_PreventAbort

Never used

* Delete DisableAbortCheckHolder

* Delete GetCurrentTaskType()

* Delete ForbidSuspendThreadCrstHolder

* Delete Thread::IsWithinCer

* Delete fOneOnly

* Move GetProcessMemoryLoad

* Delete m_CleanupIPs

* Delete 64k aliasing conflict workaround for Pentium 4

Not needed and/or included in the OS libraries

* Delete loader heap perf counters

* Delete SOWork*

* Delete ClearThrowablesForUnload