Remove redundant RTL from multiplier file

Created by: M-Ijaz-10x

Hey @zarubaf, @JeanRochCoulon

As mentioned in issue#863, the following two lines are not being used in the entire module, so they seem to be redundant:

logic [riscv::XLEN*2-1:0] mult_result;
assign mult_result   = $signed({operand_a_i[riscv::XLEN-1] & sign_a, operand_a_i}) * 
                                     $signed({operand_b_i[riscv::XLEN-1] & sign_b, operand_b_i});

Moreover, the RTL of the line number#49 is similar to the one in line number#72, as shown below:

assign mult_result_d   = $signed({operand_a_i[riscv::XLEN-1] & sign_a, operand_a_i}) *
                                        $signed({operand_b_i[riscv::XLEN-1] & sign_b, operand_b_i});

I have removed these lines form multiplier.sv file.

Thanks!