Delete 16 .js.v files whose functions already exist in generic .v (math abs/bits/cbrt/exp/floor/invtrig/sin/sinh/tan/tanh/unsafe, builtin rune/utf8, strconv f32_str placeholder, strings repeat). Port strings.repeat/repeat_string to pure V in strings.v (keep strings.c.v for the C backend); port utf8_str_len (only defined in utf8.js.v) to builtin/utf8.v so the API is preserved on all backends. wyhash and atof are intentionally untouched (see #31084).