Simplify handleIncomingSSRC

[jech]

The only functional change is that the maximum number of probed
is reduced by 1, so that simulcastProbeCount is the actual number
of packets probed.

Fixes #3330

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.98%. Comparing base (d716c9b) to head (0756081).

Files with missing lines	Patch %	Lines
peerconnection.go	92.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3333      +/-   ##
==========================================
- Coverage   85.03%   84.98%   -0.06%     
==========================================
  Files          80       80              
  Lines        9491     9496       +5     
==========================================
- Hits         8071     8070       -1     
- Misses        999     1003       +4     
- Partials      421      423       +2     

Flag	Coverage Δ
go	84.98% <92.30%> (-0.06%)	⬇️

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