There’s trick. Unicode has got a single code for ë itself, but it has also got a code which (simplified) adds ¨ on top of the previous character. The unicode standard defines a normal form in which (at least) all such characters, which can be, are represented using such modifiers. If you then simply ignore the non-ascii representable codes, you’ll get the desired result.

In python: unicodedata.normalize('NFKD', txt).encode('ASCII', 'ignore').

However, this isn’t the right solution. For instance, in german, one prefers ue as a replacement of ü over u.