Remove bash

[blueyed]

This requires to install ncurses explicitly, which was installed as a
dep for bash before, and Vim requires it.

Ref: #20 (comment)

[tweekmonster]

This looks fine to me if you know it runs. I tend to use bash because it's my safe medium for scripts 😬

[blueyed]

if you know it runs

We have quite good tests for vim-testbed already after all.
And if it breaks for users, they should add apk --no-cache add bash to their Dockerfile.

[blueyed]

Thanks @bounceme for pointing it out in #20 (comment)!