Do not return unused values from wantlists

[gammazero]

• Remove unnecessary setting empty dict to nil
• Do not return unused value from Remove or from Contains
• Do not export functions unnecessarily.

Skip changelog because there are not functional changes here.

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.46%. Comparing base (b62b60e) to head (3263926).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...tswap/client/internal/messagequeue/messagequeue.go	93.10%	0 Missing and 2 partials ⚠️

@@            Coverage Diff             @@
##             main     #792      +/-   ##
==========================================
- Coverage   60.53%   60.46%   -0.07%     
==========================================
  Files         245      245              
  Lines       31139    31132       -7     
==========================================
- Hits        18849    18823      -26     
- Misses      10615    10628      +13     
- Partials     1675     1681       +6     

Files with missing lines	Coverage Δ
...ernal/blockpresencemanager/blockpresencemanager.go	93.18% <100.00%> (-3.63%)	⬇️
bitswap/client/wantlist/wantlist.go	90.62% <100.00%> (-0.29%)	⬇️
...tswap/client/internal/messagequeue/messagequeue.go	83.39% <93.10%> (-2.05%)	⬇️

... and 8 files with indirect coverage changes

[guillaumemichel]

LGTM

The change is breaking Wantlist compatibility (Remove, Contains), but I guess there aren't many users anyway

[gammazero]

@guillaumemichel I think it is safe to assume that no one is using the bitswap/client/wantlist package outside of this code.