glsl: Clean up "unused parameter" warnings

glsl: Clean up "unused parameter" warnings

../../src/glsl/ir_constant_expression.cpp:486:1: warning: unused parameter 'variable_context' [-Wunused-parameter]
../../src/glsl/ir_constant_expression.cpp:1633:1: warning: unused parameter 'variable_context' [-Wunused-parameter]
../../src/glsl/ir_constant_expression.cpp:1752:1: warning: unused parameter 'variable_context' [-Wunused-parameter]
../../src/glsl/ir_constant_expression.cpp:1761:1: warning: unused parameter 'variable_context' [-Wunused-parameter]
../../src/glsl/ir_constant_expression.cpp:1769:1: warning: unused parameter 'variable_context' [-Wunused-parameter]

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>