remove importmap shims

[aseroff]

Context

Fixes #1358

Changes proposed in this pull request

Remove importmap shims entirely.

Guidance to review

Alternatives considered:

• checking importmap gem version in method
• calling defined? in ternary expression
• pinning Spina's importmap version to v1

[Bramjetten]

Thanks for fixing this so quickly :)

I’ll release a new version of Spina today, without importmap shims.