[OpenMP][Docs] Remove old/outdated webpage

This should have happened a long time ago, now that openmp.llvm.org
redirects to openmp.llvm.org/docs we completely switched over to the
sphinx documentation page instead.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D108588

GitOrigin-RevId: 9f844aeeb42be1b0d14b01531a543baf869f896d
5 files changed