cpp_generator::print_method_param_use: simplify condition
cpp_generator::print_method_param_use: simplify condition

The is_callback case is already handled earlier in the function,
meaning that !is_callback always holds.
The extra test was already present when the code was
introduced in isl-0.18-599-g7d3c260fb5 (cpp: support methods and
constructors, Thu Apr 13 21:42:03 2017 +0200) and
only causes confusion.  Remove it.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl