I cant remove / drop unique index key from mysql table

hello , I have this migration -

 Schema::create('my_table', function (Blueprint $table) {
			$table->bigIncrements('id');

            $table->bigInteger('post_id')->unsigned();
            $table->foreign('post_id')->references('id')->on('prop_new')->onDelete('cascade');
		    $table->string('title', 255);
 		   $table->unique(['post_id','title']);
 });

 public function down()
    {
        Schema::table('my_table', function(Blueprint $table){
            $table->dropForeign('my_table_post_id_foreig');

        });
        Schema::dropIfExists('my_table');
    }

    }

Laravel creates a unique key in for post_id and title , called -

my_table_post_id_title_unique

in table - my_table .

If you go to phpmyadmin when you migrate this , you gonna see this key there , on Indexes . I tried to delete this unique key index . but I am not able . it says this message -

#1553 - Cannot drop index 'my_table_post_id_title_unique': needed in a foreign key constraint

I tried as well make those commands in mysql -

ALTER TABLE `my_table` DROP INDEX `my_table_post_id_title_unique`;

or 

DROP INDEX `my_table_post_id_title_unique` ON `my_table`

But I was no success , I cant drop the unique index key .