v_ext_macros: Drop index from `VI_ELEMENT_SKIP` (#1151)

If a variable other than `i` is passed as `inx`, `VI_LOOP_ELEMENT_SKIP`
will still check `i` in the `else` case, ignoring `inx`.

Since `i` is the only value ever passed, drop the argument to avoid any
future confusion.

Signed-off-by: Dylan Reid <dgreid@rivosinc.com>

Signed-off-by: Dylan Reid <dgreid@rivosinc.com>