Remove mention of other vector image editors in various places

There was a discussion in the website channel about mentioning other/proprietary applications that do similar things to Inkscape.

There is some concern that constantly comparing ourselves to Illustrator in every description of Inkscape, kind of makes us look like we're too weak to stand on our own or too uncertain of our own brand and offering. Or that we seek to be a "drop-in" replacement for some of these tools. Even though we do things that Illustrator and Corel do not.

This merge request removes some of the text we currently have. You can see from the diff that many of the locations this text has been mutated and changed and it a bit inconsistent. But it also should be asked if advertising what we're not in every description is something we want to do.

This is an RFC, there's no mind to put this into 1.0 (string changes!) and the more developers can related their thoughts the better.