#113 Remove xpatch dependency from scrextend

[Cabohah]

Currently package scrextend loads xpatch if \@makefnmark differs from the definition used by the standard classes. But IMHO this is not really needed. If users want to use \deffootnote and \deffootnotemark the \deffootnotemark already redefines \@makefnmark.

Usage of xpatch can result in issues, because it it incompatible with the new regexpatch package. → https://tex.stackexchange.com/questions/767439/incompatibility-between-amsart-scrextend-and-regexpatch

[Markus Kohm]

\deffootnote does not run \deffootnotemark and therefore also does not redefine \@makefnmark.

Nevertheless, such a change was easy to do adding extendedfeature=footnote.

This is a breaking change sometimes requested by users. Without the option scrextend does no longer use \deffootnotemark at load time. Not running \deffootnote at load time was already the case before. So, now several features are only available if you use the option or do \deffootnote and \deffootnotemark on your own. This makes scrextend less invasive.

See [r4329] (implementation) and [r4331] (documentation).