Remove the code that determined LHS/RHS context for var expressions
    (was needed previously for some instances of the back end). LHS/RHS
    context is computed by the back end in all cases, so no need to have
    this code in the front end any more.
    
    Reviewed-on: https://go-review.googlesource.com/77510

	* go-gcc.cc (var_expression): Remove Varexpr_context parameter.

From-SVN: r254748