don't send path MTU probe packets on a timer

[marten-seemann]

Fixes #3409.

[codecov]

Codecov Report

Merging #3423 (a921f23) into master (60bbe92) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3423      +/-   ##
==========================================
+ Coverage   85.37%   85.39%   +0.01%     
==========================================
  Files         135      135              
  Lines        9928     9922       -6     
==========================================
- Hits         8476     8472       -4     
+ Misses       1067     1066       -1     
+ Partials      385      384       -1     

Impacted Files	Coverage Δ
connection.go	77.48% <ø> (+0.12%)	⬆️
mtu_discoverer.go	100.00% <100.00%> (ø)

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