Use fisher to install, update, and remove fisher

[jorgebucaran]

Remove self-update and self-uninstall. Instead use fisher to install, update, or remove itself as every other plugin. Inspired by @IlanCosman work on Scuba. #594

This does not only help simplify the code base, but also make bootstrapping fisher more powerful by making the fisher function immediately available upon installation. This also allows us to include completions separately, rather than embedding them in the same file.

curl -sL git.io/fisher | source && fisher install jorgebucaran/fisher

We no longer need self-update and self-uninstall either since we can just:

fisher update fisher

or to update everything (including fisher):

fisher update

and

fisher remove fisher

although I'd prefer to remove all plugins as well, since that's what self-uninstall used to do—no problem:

fisher list | fisher remove

[jorgebucaran]

Implemented in 4.x. :tada: