Fix deadlock

[pepicrft]

Resolves #446

Short description 📝

The delete from PBXObjects, does not use the lock properly and as a consequence, we end up with a dead lock.

Solution 📦

Add a defer block to unlock the lock.

[codecov]

Codecov Report

Merging #449 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
+ Coverage   80.42%   80.43%   +0.01%     
==========================================
  Files         149      149              
  Lines        7652     7652              
==========================================
+ Hits         6154     6155       +1     
+ Misses       1498     1497       -1

Impacted Files	Coverage Δ
Sources/xcodeproj/Objects/Project/PBXObjects.swift	73.28% <100%> (+0.36%)	⬆️

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