Documentation/DocBook: remove gpu.tmpl

The gpu documentation has now been converted to reStructuredText files
under Documentation/gpu. Remove the obsolete DocBook template. Also
remove it from MAINTAINERS.

Good riddance.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/8d673f75fe686371ed9838682c368a4e3b96bf54.1466506505.git.jani.nikula@intel.com