Open Preferences in TextEdit. Under "New Document" check off "Plain Text" under Format. Under "Open and Save" make sure "Add '.txt' extension to plain text files" is unchecked.
New files will be in plain text and when you save them you can use any extension you want.
To change an existing rtf file to plain text. Press Command-A to select everything in the file and then go to the Format menu and choose "Make Plain Text". Save the file.
Having an htm(l) extension is as much for human convenience as it is for a browser. Having an html extension will allow a file to be loaded into a browser by default, but if you open a file, regardless of extension, using "Open File" in a browser, then if it is properly formated html code, the browser will render it as a web page.