glsl2: Remove incorrect assertion in the parser.

glsl2: Remove incorrect assertion in the parser.

This assertion is triggered by method calls (i.e. array.length()), where
subexpressions[1] is an ast_function_call expression.  Since the
assertion itself had a comment saying it could be removed eventually,
simply do so.

Causes negative glslparser tests array-length-110.frag,
array-length-args.frag, and array-length-unsized.frag to pass, but only
because the length() method is not supported yet.