clang-tidy: Fix all remaining misc-include-cleaner violations

- Remove unused includes
- Add common false positive headers to misc-include-cleaner.IgnoreHeaders
- Add IWYU pragma keep for uncommon false positive headers

src/systemctl/systemctl-show.c

@@ -17,7 +17,6 @@
 #include "errno-util.h"
 #include "exec-util.h"
 #include "exit-status.h"
-#include "extract-word.h"
 #include "format-util.h"
 #include "hexdecoct.h"
 #include "hostname-setup.h"