[c++] Remove some dead code in int64-only shape accessor

[johnkerl]

Issue and/or context: Found at https://github.com/single-cell-data/TileDB-SOMA/pull/2911/files#r1721037749 on PR #2911, for issue #2407 / [sc-51048].

Changes:

We were capable of returning incorrect information for arrays which do not have int64_t dimensions. However, passing status on all Python/R-level unit tests shows that this is a safe and non-breaking change.

Notes for Reviewer:

This PR should be approved and merged before #2911. Then #2915 can be rebased on top of the new main.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.03%. Comparing base (6f88d57) to head (a65cbd7).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2915      +/-   ##
==========================================
+ Coverage   89.88%   90.03%   +0.15%     
==========================================
  Files          37       37              
  Lines        3925     3925              
==========================================
+ Hits         3528     3534       +6     
+ Misses        397      391       -6     

Flag	Coverage Δ
python	90.03% <ø> (+0.15%)	⬆️

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

Components	Coverage Δ
python_api	90.03% <ø> (+0.15%)	⬆️
libtiledbsoma	∅ <ø> (∅)

[johnkerl]

Thanks @nguyenv ! :)