fix: Use of SentrySpan as class

[noahsmartin]

This deletes an unused "SpanProtocol" that I had accidentally left in from a previous PR (#6274)

It also corrects the usage of SentrySpan in a forward declaration that I happened to notice while working on another Swift conversion

#skip-changelog

Closes #6416

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.860%. Comparing base (4f36ea5) to head (955e552).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #6415       +/-   ##
=============================================
+ Coverage   86.838%   86.860%   +0.021%     
=============================================
  Files          441       441               
  Lines        37261     37261               
  Branches     17339     17344        +5     
=============================================
+ Hits         32357     32365        +8     
+ Misses        4859      4853        -6     
+ Partials        45        43        -2     

Files with missing lines	Coverage Δ
...egrations/Performance/IO/SentryFileIOTracker.swift	100.000% <ø> (ø)

... and 8 files with indirect coverage changes

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1219.91 ms	1256.46 ms	36.54 ms
Size	23.75 KiB	994.81 KiB	971.07 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
fc05805	1220.63 ms	1252.16 ms	31.54 ms
b9aacb6	1230.42 ms	1251.00 ms	20.58 ms
d8b2303	1231.53 ms	1249.89 ms	18.35 ms
1339919	1214.82 ms	1238.98 ms	24.16 ms
ac4739e	1236.55 ms	1258.89 ms	22.34 ms
b714cb9	1224.71 ms	1238.04 ms	13.33 ms
d157d83	1228.02 ms	1252.47 ms	24.45 ms
aff3b66	1229.53 ms	1263.08 ms	33.55 ms
7d88965	1228.86 ms	1248.53 ms	19.67 ms
30f4e4c	1213.21 ms	1250.29 ms	37.08 ms

App size

Revision	Plain	With Sentry	Diff
fc05805	23.75 KiB	908.02 KiB	884.27 KiB
b9aacb6	23.75 KiB	913.64 KiB	889.89 KiB
d8b2303	23.75 KiB	908.02 KiB	884.27 KiB
1339919	23.75 KiB	919.70 KiB	895.95 KiB
ac4739e	23.75 KiB	872.67 KiB	848.92 KiB
b714cb9	23.75 KiB	858.69 KiB	834.93 KiB
d157d83	23.75 KiB	928.85 KiB	905.10 KiB
aff3b66	23.75 KiB	978.53 KiB	954.78 KiB
7d88965	23.75 KiB	994.72 KiB	970.98 KiB
30f4e4c	23.75 KiB	879.24 KiB	855.50 KiB

[philipphofmann]

LGTM