chore(nano): remove deprecated get_balance

[glevco]

Motivation

The get_balance syscall was deprecated in favor of the explicit methods get_current_balance and get_balance_before_current_call. This PR removes it.

Originally the task was to remove the *_before_current_call syscalls. But I gave more thought into it and I think it's good to keep them both, as it makes it clear for the user that there are different ways of interpreting the balance, and that they should choose the right one for their use case. Otherwise, it requires more knowledge of how our system works (for example, knowing when actions are executed).

Acceptance Criteria

• Remove get_balance syscall.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	chore/nano/remove-deprecated
Testbed	ubuntu-22.04

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Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.65 m (+0.43%) Baseline: 1.64 m	1.47 m (89.62%)	1.80 m (91.30%)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.58%. Comparing base (3c4b408) to head (62d0d5b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
- Coverage   85.61%   85.58%   -0.03%     
==========================================
  Files         424      424              
  Lines       31994    31986       -8     
  Branches     4971     4967       -4     
==========================================
- Hits        27391    27375      -16     
- Misses       3606     3608       +2     
- Partials      997     1003       +6     

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