Remove SentryTimingsCallback and use Flutter's TimingsCallback instead

[buenaflor]

📜 Description

We have since bumped our Flutter version so we can safely use it now

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (fd4d716) to head (2580b41).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3054       +/-   ##
===========================================
+ Coverage   77.59%   89.82%   +12.23%     
===========================================
  Files          25       96       +71     
  Lines         656     3331     +2675     
===========================================
+ Hits          509     2992     +2483     
- Misses        147      339      +192     

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[github-actions]

iOS Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1255.39 ms	1263.29 ms	7.90 ms
Size	7.86 MiB	9.44 MiB	1.58 MiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
0fb3800	1256.60 ms	1266.28 ms	9.68 ms
2d34233	1258.19 ms	1268.92 ms	10.73 ms
aeb02f2	1244.29 ms	1256.55 ms	12.26 ms
6ba4675	1223.12 ms	1238.17 ms	15.04 ms
e2d675d	1238.48 ms	1242.76 ms	4.28 ms
9b99523	1256.06 ms	1270.33 ms	14.27 ms
ec78888	1251.37 ms	1269.40 ms	18.04 ms
b6c8720	1252.65 ms	1266.61 ms	13.96 ms
640ad0c	1241.04 ms	1253.96 ms	12.92 ms
827bf09	1261.86 ms	1276.41 ms	14.55 ms

App size

Revision	Plain	With Sentry	Diff
0fb3800	7.86 MiB	9.44 MiB	1.58 MiB
2d34233	7.86 MiB	9.44 MiB	1.58 MiB
aeb02f2	7.86 MiB	9.44 MiB	1.58 MiB
6ba4675	7.86 MiB	9.44 MiB	1.58 MiB
e2d675d	7.86 MiB	9.44 MiB	1.58 MiB
9b99523	7.86 MiB	9.44 MiB	1.58 MiB
ec78888	7.86 MiB	9.44 MiB	1.58 MiB
b6c8720	7.86 MiB	9.44 MiB	1.58 MiB
640ad0c	7.86 MiB	9.44 MiB	1.58 MiB
827bf09	7.86 MiB	9.44 MiB	1.58 MiB

Previous results on branch: refactor/use-timings-callback

Startup times

Revision	Plain	With Sentry	Diff
44c55bb	1265.81 ms	1274.52 ms	8.71 ms

App size

Revision	Plain	With Sentry	Diff
44c55bb	7.86 MiB	9.44 MiB	1.58 MiB