refactor: Remove non-functional `package.json#exports.umd`

[rschristian]

In all three of the packages, package.json#exports.umd has been referencing the incorrect path for the UMD bundles since release (should've been x.min.js).

However, seeing as how I can't see a single complaint relating to this, and I'm not aware of any tools that actually consume it (would love to know if there are tools that do), I elected to remove the entries. Saves us from making a breaking change in the future if we were to alter that entry in any way.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 878245c44c494349fe1330c3986524d2e4e39551

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

| Name | Type | | --------------------- | ----- | | @preact/signals-core | Patch | | @preact/signals | Patch | | @preact/signals-react | Patch |

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[github-actions[bot]]

Size Change: 0 B

Total Size: 69.1 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `docs/dist/assets/client.********.js` | 46.6 kB | | `docs/dist/assets/index.********.js` | 835 B | | `docs/dist/assets/jsxRuntime.module.********.js` | 282 B | | `docs/dist/assets/preact.module.********.js` | 4 kB | | `docs/dist/assets/signals-core.module.********.js` | 1.42 kB | | `docs/dist/assets/signals.module.********.js` | 1.97 kB | | `docs/dist/assets/style.********.js` | 21 B | | `docs/dist/assets/style.********.css` | 1.21 kB | | `docs/dist/basic-********.js` | 244 B | | `docs/dist/demos-********.js` | 3.35 kB | | `docs/dist/nesting-********.js` | 1.13 kB | | `docs/dist/react-********.js` | 239 B | | `packages/core/dist/signals-core.js` | 1.48 kB | | `packages/core/dist/signals-core.mjs` | 1.5 kB | | `packages/preact/dist/signals.js` | 1.2 kB | | `packages/preact/dist/signals.mjs` | 1.15 kB | | `packages/react/dist/signals.js` | 1.25 kB | | `packages/react/dist/signals.mjs` | 1.21 kB |

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[rschristian]

Can do! I should've mentioned this on that PR, but evidently forgot to.