Use Multiplication unicode symbol for delete

[urre]

This PR replaces the usage of a normal "x" text with the Unicode Character “✕” (U+2715) instead. It looks better to represent "close" or "remove".

[jerosoler]

Thanks for the PR.

Currently I am only fixing bug and I am not adding changes.

[skahmad]

1. add stylesheet in src/drawflow.css

.drawflow-delete span {
position: relative;
left: -8px;
top: 5px;
}

2. change line number 533 under context menu method to the blow code
   from : deletebox.innerHTML = "x";
   to: deletebox.innerHTML = "";