doc: Makefile: Remove latex_paper_size (PAPER) option
doc: Makefile: Remove latex_paper_size (PAPER) option

This configuration variable was removed in Sphinx 1.6:
http://www.sphinx-doc.org/en/master/changes.html#id35. I suspect no one
might be using it. It could still be passed in SPHINXOPTS if needed.

Maybe the paper size should be set from Python instead:
http://www.sphinx-doc.org/en/master/usage/configuration.html

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

kraj/zephyr,explora26/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,mbolivar/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,mbolivar/zephyr,mbolivar/zephyr,galak/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,kraj/zephyr,ldts/zephyr