simplify x = x <op> ... to x <op>= ...

[ludweeg]

Found using https://go-critic.github.io/overview#assignOp-ref

[codecov-io]

Codecov Report

Merging #1538 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1538   +/-   ##
=======================================
  Coverage   89.05%   89.05%           
=======================================
  Files         123      123           
  Lines        8610     8610           
=======================================
  Hits         7667     7667           
  Misses        642      642           
  Partials      301      301

Impacted Files	Coverage Δ
internal/congestion/cubic_sender.go	92.48% <100%> (ø)	⬆️
internal/ackhandler/sent_packet_handler.go	80.23% <100%> (ø)	⬆️

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