Allow update existing stake pool when at max delegates

[Sriep]

Fixes

Allows a delegate to lock tokens to an existing pool, when the stake pool has the maximum number of delegates.
Removes some dead code.

Changes

Need to be mentioned in CHANGELOG.md?

Tests

Tasks to complete before merging PR:

• Ensure system tests are passing. If not Run them manually to check for any regressions 📋
• Do any new system tests need added to test this change? do any existing system tests need updated? If so create a PR at 0chain/system_test
• Merge your system tests PR to master AFTER merging this PR

Associated PRs (Link as appropriate):

• blobber:
• gosdk:
• system_test:
• zboxcli:
• zwalletcli:
• Other: ...

[codecov-commenter]

Codecov Report

Merging #1832 (b8317c7) into staging (9b5e165) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           staging    #1832   +/-   ##
========================================
  Coverage    26.42%   26.43%           
========================================
  Files          355      355           
  Lines        59067    59063    -4     
========================================
+ Hits         15608    15611    +3     
+ Misses       41559    41550    -9     
- Partials      1900     1902    +2     

Flag	Coverage Δ
Unit-Tests	26.43% <0.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...0chain.net/smartcontract/minersc/delegate_pools.go	0.00% <0.00%> (ø)
...go/0chain.net/smartcontract/stakepool/stakepool.go	18.12% <0.00%> (+0.21%)	⬆️
...go/0chain.net/smartcontract/storagesc/stakepool.go	34.59% <0.00%> (ø)
...ode/go/0chain.net/smartcontract/zcnsc/stakepool.go	35.66% <0.00%> (ø)
...ain.net/smartcontract/storagesc/config_settigns.go	54.76% <0.00%> (ø)
...de/go/0chain.net/smartcontract/vestingsc/config.go	50.52% <0.00%> (+1.57%)	⬆️

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[peterlimg]

LGTM