Revert "Reintroduce special handling for Windows' inability to release memory-mapped files."

(And the subsequent attempts to sort out licencing problems.)

This reverts commit dee749643f2d4cfa34d7331426e6ae890aebdb3b.
This reverts commit 9fa7eae82fdcb6a9bb91976674af7907aa17123c.
This reverts commit cdb769a39918a18dffc1fe58021e8ea793016f50.
This reverts commit f32874a05754dbb793737d4fd86cc9bb218416df.
This reverts commit afc223397ab6205e5a1fb9edc35d2967f7830114.
This reverts commit c9a0cc1157c5b0f4eb43a671a5e4c65dbc0ce985.