Remove unnecessary unfetch polyfill for dev

[Timer]

Fetch is always polyfilled in legacy browsers by @next/polyfill-nomodule, so we do not need to import unfetch for IE11 support.

Fixes #20588

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
buildDuration	9.5s	9.5s	-66ms
nodeModulesSize	82.9 MB	82.9 MB	-1.46 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
/ failed reqs	0	0	✓
/ total time (seconds)	1.955	1.974	⚠️ +0.02
/ avg req/sec	1278.97	1266.75	⚠️ -12.22
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.398	1.413	⚠️ +0.02
/error-in-render avg req/sec	1788.34	1769.63	⚠️ -18.71

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
677f882d2ed8..45a8.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-eda9847..6ba4.js gzip	6.56 kB	6.56 kB	✓
webpack-7193..1446.js gzip	751 B	751 B	✓
Overall change	59 kB	59 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
polyfills-67..b7d1.js gzip	31.2 kB	31.2 kB	✓
Overall change	31.2 kB	31.2 kB	✓

Client Pages

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
_app-370a053..c381.js gzip	1.28 kB	1.28 kB	✓
_error-b7502..6249.js gzip	3.44 kB	3.44 kB	✓
hooks-baa922..722d.js gzip	887 B	887 B	✓
index-c8a663..7961.js gzip	227 B	227 B	✓
link-64502f2..a071.js gzip	1.61 kB	1.61 kB	✓
routerDirect..4623.js gzip	303 B	303 B	✓
withRouter-f..7105.js gzip	302 B	302 B	✓
Overall change	8.05 kB	8.05 kB	✓

Client Build Manifests

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
_buildManifest.js gzip	323 B	323 B	✓
Overall change	323 B	323 B	✓

Rendered Page Sizes

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
index.html gzip	614 B	614 B	✓
link.html gzip	620 B	620 B	✓
withRouter.html gzip	608 B	608 B	✓
Overall change	1.84 kB	1.84 kB	✓

---

Serverless Mode (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
buildDuration	11.4s	11.3s	-36ms
nodeModulesSize	82.9 MB	82.9 MB	-1.46 kB

Client Bundles (main, webpack, commons)

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
677f882d2ed8..45a8.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-eda9847..6ba4.js gzip	6.56 kB	6.56 kB	✓
webpack-7193..1446.js gzip	751 B	751 B	✓
Overall change	59 kB	59 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
polyfills-67..b7d1.js gzip	31.2 kB	31.2 kB	✓
Overall change	31.2 kB	31.2 kB	✓

Client Pages

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
_app-370a053..c381.js gzip	1.28 kB	1.28 kB	✓
_error-b7502..6249.js gzip	3.44 kB	3.44 kB	✓
hooks-baa922..722d.js gzip	887 B	887 B	✓
index-c8a663..7961.js gzip	227 B	227 B	✓
link-64502f2..a071.js gzip	1.61 kB	1.61 kB	✓
routerDirect..4623.js gzip	303 B	303 B	✓
withRouter-f..7105.js gzip	302 B	302 B	✓
Overall change	8.05 kB	8.05 kB	✓

Client Build Manifests

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
_buildManifest.js gzip	323 B	323 B	✓
Overall change	323 B	323 B	✓

Serverless bundles Overall increase ⚠️

	vercel/next.js canary	Timer/next.js remove-dep/unfetch	Change
_error.js	1 MB	1 MB	⚠️ +32 B
404.html	2.67 kB	2.67 kB	✓
hooks.html	1.92 kB	1.92 kB	✓
index.js	1 MB	1 MB	⚠️ +32 B
link.js	1.06 MB	1.06 MB	⚠️ +32 B
routerDirect.js	1.05 MB	1.05 MB	⚠️ +32 B
withRouter.js	1.05 MB	1.05 MB	⚠️ +32 B
Overall change	5.16 MB	5.16 MB	⚠️ +160 B

Commit: de14863