fix(zsh): drop redundant awk pipeline in language completion

`bat --list-languages` already emits each entry in `name:matchers`
form, which is the format `_describe` consumes directly. The previous
awk script split each line on `:` and re-emitted `$1:$2`, which is
byte-identical to the input.

Verified with `diff <(bat --list-languages) <(bat --list-languages |
awk -F: '{ printf("%s:%s\\n", $1, $2) }')` against the current
syntax set.