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 The C ftplugin is obsolete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-=-

.config/vim/vimrc.d/30-settings.vim

@@ -93,8 +93,6 @@ set shortmess+=A
 " probably expected. See corresponding autocommand or:
 " https://vi.stackexchange.com/questions/2702
 set autoread
-" Include `-` in keyword characters
-set iskeyword+=-
 " Do not automatically insert <EOL> at EOF if missing
 set nofixendofline
 " Let the cursor move beyond the EOL when in visual-block mode.