Clean up distinguished but equivalent path in vmid_to_ipv4

[minitech]

For all vmids from 1 to 254, (vmid >> 8, vmid & 0xff) == ((vmid - 1) // 254, (vmid - 1) % 254 + 1) == (0, vmid).

(Happened to come across this while working on something else and figured I’d put the patch up.)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.35%. Comparing base (125cb5a) to head (7d9bdbd).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
qubes/vm/mix/net.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #823      +/-   ##
==========================================
- Coverage   70.42%   70.35%   -0.07%     
==========================================
  Files          61       61              
  Lines       14112    14086      -26     
==========================================
- Hits         9938     9910      -28     
- Misses       4174     4176       +2     

Flag	Coverage Δ
unittests	70.35% <50.00%> (-0.07%)	⬇️

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