landlock: remove empty functions

Functions with `...` as the first parameter appear to be unsupported in
older versions of gcc, as they fail to compile.  Examples:

Error from gcc 9.5.0-1ubuntu1~16.04.sav1 on Ubuntu 16.04:

    [...]
    In file included from appimage.c:23:
    firejail.h:981:27: error: ISO C requires a named argument before ‘...’
      981 | static inline int ll_read(...) { return 0; }
          |                           ^~~

Warning from gcc 13.2.1-3 on Artix Linux:

    $ ./configure --disable-landlock >/dev/null && make clean >/dev/null &&
      make EXTRA_CFLAGS+='-std=c99 -Wpedantic -Wno-error'
    [...]
    gcc -ggdb -O2 -DVERSION='"0.9.73"' -DMOD_DIR='"src/firejail"' [...]
    In file included from appimage.c:23:
    firejail.h:982:27: warning: ISO C requires a named argument before ‘...’ before C2X [-Wpedantic]
      982 | static inline int ll_read(...) { return 0; }
          |                           ^~~

Fixes #6115.

Relates to #6078.

src/firejail/firejail.h

@@ -976,16 +976,6 @@ int ll_exec(const char *allowed_path);
 int ll_basic_system(void);
 int ll_restrict(__u32 flags);
 void ll_add_profile(int type, const char *data);
-#else
-static inline int ll_get_fd(void) { return -1; }
-static inline int ll_is_supported(void) { return 0; }
-static inline int ll_read(...) { return 0; }
-static inline int ll_write(...) { return 0; }
-static inline int ll_special(...) { return 0; }
-static inline int ll_exec(...) { return 0; }
-static inline int ll_basic_system(void) { return 0; }
-static inline int ll_restrict(...) { return 0; }
-static inline void ll_add_profile(...) { return; }
 #endif /* HAVE_LANDLOCK */
 
 #endif