Trivial: Remove `TransactionRelayerFeeImpl` unnecessary ctor field

[mkykadir]

start_timers field was only used for the module unittest; this test doesn't invoke the start anyway.

[codecov[bot]]

Codecov Report

Merging #3305 (c2f29ec) into v0.x.x (be9b8d1) will increase coverage by 0.73%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           v0.x.x    #3305      +/-   ##
==========================================
+ Coverage   87.83%   88.57%   +0.73%     
==========================================
  Files         164      165       +1     
  Lines       16964    17072     +108     
==========================================
+ Hits        14901    15122     +221     
+ Misses       2063     1950     -113     

Flag	Coverage Δ
integration	38.30% <66.66%> (?)
unittests	87.84% <66.66%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/node/TransactionRelayer.d	89.09% <66.66%> (-0.20%)	:arrow_down:
source/agora/common/Task.d	88.88% <0.00%> (-11.12%)	:arrow_down:
source/agora/flash/Node.d	80.67% <0.00%> (-0.43%)	:arrow_down:
source/agora/node/main.d	44.85% <0.00%> (ø)
source/agora/script/Engine.d	97.49% <0.00%> (+0.14%)	:arrow_up:
source/agora/consensus/protocol/Nominator.d	92.00% <0.00%> (+0.35%)	:arrow_up:
source/agora/node/Validator.d	92.46% <0.00%> (+0.54%)	:arrow_up:
source/agora/node/Registry.d	79.37% <0.00%> (+2.08%)	:arrow_up:
source/agora/node/FullNode.d	75.85% <0.00%> (+3.09%)	:arrow_up:
source/agora/utils/Log.d	62.50% <0.00%> (+5.26%)	:arrow_up:
... and 4 more

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