[R-D]

Looks to me like you need to edit clientscript/textedit.js. There's 4 references to http:// there, one for images and three for links. Not sure whether you need to remove all 4 references but I just tested and that seemed to work. Note: an F5 refresh was needed to clear the old file from my browser's cache.

Find:

img = this.show_prompt(vbphrase['enter_image_url'], 'http://', true);

Change to:

img = this.show_prompt(vbphrase['enter_image_url'], '', true);

Find (twice):

url = this.show_prompt(vbphrase['enter_link_url'], 'http://', true);

and change both to:

url = this.show_prompt(vbphrase['enter_link_url'], '', true);

Find:

this.prompt_link('url', url, vbphrase['enter_link_url'], 'http://');

and change to:

this.prompt_link('url', url, vbphrase['enter_link_url'], '');