refactor: inline get_epoch_minted_amount in epoch manager

[stedfn]

This PR inlines the usage of get_epoch_minted_amount and removes its definition as it is only used in 2 places

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.46%. Comparing base (c10b078) to head (14aa7bd).

Files with missing lines	Patch %	Lines
chain/chain/src/chain.rs	0.00%	0 Missing and 1 partial ⚠️
chain/client/src/client.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12435      +/-   ##
==========================================
- Coverage   71.47%   71.46%   -0.02%     
==========================================
  Files         837      837              
  Lines      169397   169390       -7     
  Branches   169397   169390       -7     
==========================================
- Hits       121076   121047      -29     
- Misses      42976    42991      +15     
- Partials     5345     5352       +7     

Flag	Coverage Δ
backward-compatibility	0.16% <0.00%> (+<0.01%)	⬆️
db-migration	0.16% <0.00%> (+<0.01%)	⬆️
genesis-check	1.28% <0.00%> (+<0.01%)	⬆️
integration-tests	39.35% <0.00%> (-0.01%)	⬇️
linux	70.78% <0.00%> (+<0.01%)	⬆️
linux-nightly	71.03% <0.00%> (-0.01%)	⬇️
macos	51.04% <0.00%> (-0.01%)	⬇️
pytests	1.59% <0.00%> (+<0.01%)	⬆️
sanity-checks	1.40% <0.00%> (+<0.01%)	⬆️
unittests	64.01% <0.00%> (-0.01%)	⬇️
upgradability	0.21% <0.00%> (+<0.01%)	⬆️

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