refactor(adb): remove outdated completion plugin adb (ohmyzsh#12576)

BREAKING CHANGE: the `adb` plugin has been removed in favor of the completion
provided out-of-the-box by zsh since 4.3.14.

This commit removes plugin `adb`, which bundles custom completion for the
`adb` tool for Android. `adb` completion is already supported out of the
box in zsh 4.3.14 and older.

Reference: zsh-users/zsh@d1a557d