feat(nextjs): remove tracing from pages router API routes

[logaretm]

This PR removes tracing logic from Pages Router API routes template (non-edge), completely switching our reliance to the spans created and emitted by Next.js.

[linear]

JS-1209 Remove tracing from Pages Router API handler template

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	9,406	-	9,185	+2%
GET With Sentry	1,794	19%	1,801	-0%
GET With Sentry (error only)	6,293	67%	6,271	+0%
POST Baseline	1,200	-	1,203	-0%
POST With Sentry	604	50%	604	-
POST With Sentry (error only)	1,075	90%	1,074	+0%
MYSQL Baseline	3,427	-	3,372	+2%
MYSQL With Sentry	509	15%	465	+9%
MYSQL With Sentry (error only)	2,771	81%	2,751	+1%

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