DOC: Update HOWTO_RELEASE.rst
DOC: Update HOWTO_RELEASE.rst

Update HOWTO_RELEASE.rst and remove sections that are duplicated
in the RELEASE_WALKTHROUGH.rst file.

mhvk/numpy,endolith/numpy,mattip/numpy,rgommers/numpy,charris/numpy,charris/numpy,anntzer/numpy,numpy/numpy,charris/numpy,mattip/numpy,mattip/numpy,charris/numpy,mhvk/numpy,rgommers/numpy,mattip/numpy,mhvk/numpy,mhvk/numpy,endolith/numpy,mhvk/numpy,numpy/numpy,endolith/numpy,rgommers/numpy,anntzer/numpy,numpy/numpy,rgommers/numpy,endolith/numpy,anntzer/numpy,anntzer/numpy,numpy/numpy