MAINT: remove some unused code from multipack.h headers.

Pretty much the same code in integrate and interpolate.
MAINT: remove unused check_args function from odrpack.
DOC: INSTALL: use Py3-compatible commands, and don't mention Py2.4
Merge pull request #4255 from larsmans/doc-maint

DOC: INSTALL: use Py3-compatible print syntax, and don't mention Py2.4
Merge pull request #4247 from rgommers/remove-unused-code

MAINT: remove some unused code
merging upstream