[CMake,LaTeX] Revert "Allow to disable latex."

Commit 4878943f actuall did change
the default behavior, it effectifly disabled LaTex for some users.
To disable LaTeX pass -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE

(cherry picked from commit 88bbf966)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>

cmake/modules/DuneDoc.cmake

@@ -21,7 +21,7 @@ FIND_PACKAGE(LATEX)
 FIND_PROGRAM(IMAGEMAGICK_CONVERT convert
   DOC "The convert program that comes with ImageMagick (available at http://www.imagemagick.org)."
   )
-option(LATEX_USABLE "ON")
+set(LATEX_USABLE "ON")
 
 if(NOT LATEX_COMPILER)
   message(WARNING " Need latex to create documentation!")