vim:setts: Revert "Include - in &iskeyword"

Commit 0369218114 ("vim:settings: Include `-` in `&iskeyword`") added
the dash to `&iskeyword` but I didn't give an explanation for the change
and don't remember what led me to it. Today I'm annoyed by it since it
breaks some stuff (e.g. previously the C pointer arrow and syntax
highlighting of negative numbers in Python). The C ftplugin is obsolete
now, so delete it.

This reverts commit 0369218114.

.config/vim/ftplugin/c.vim

@@ -1,2 +0,0 @@
-" Exclude `-` in keyword characters
-setlocal iskeyword-=-