docs: Remove Fig instructions

[z0rc]

This project is dead. https://fig.io/blog/post/fig-is-sunsetting

Effectively it's a revert of https://github.com/zsh-users/zsh-syntax-highlighting/commit/b2c910a85ed84cb7e5108e7cb3406a2e825a858f.

[brendanfalk]

To clarify, project isn't "dead", it's being merged into CodeWhisperer for command line. However, yes, the plugins feature is not being merged into CodeWhisperer. We were planning on doing a review of all the readmes where Fig was listed removing it, however, not for about 3 months. Fig is being sunset on 1 september and will be fully supported until then.

Nonetheless, your call on when you want to remove this