Removed unneeded Default constraint

[hohav]

I was trying to use MutableListArray with a type that doesn't implement Default, and ran into an error caused by this constraint. It looks unnecessary, and removing it doesn't seem to break anything.

[codecov]

Codecov Report

Merging #1157 (47132b3) into main (1d52cbf) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1157      +/-   ##
==========================================
+ Coverage   83.66%   83.68%   +0.01%     
==========================================
  Files         365      365              
  Lines       35865    35865              
==========================================
+ Hits        30006    30013       +7     
+ Misses       5859     5852       -7     

Impacted Files	Coverage Δ
src/array/list/mutable.rs	76.04% <ø> (ø)
src/chunk.rs	83.33% <0.00%> (-7.15%)	⬇️
src/io/ipc/read/file_async.rs	60.44% <0.00%> (-0.38%)	⬇️
src/buffer/immutable.rs	89.33% <0.00%> (ø)
src/io/ipc/read/reader.rs	96.32% <0.00%> (ø)
src/io/ipc/read/stream_async.rs	76.02% <0.00%> (ø)
src/array/utf8/mod.rs	86.58% <0.00%> (+0.95%)	⬆️
src/io/ipc/read/array/fixed_size_binary.rs	94.73% <0.00%> (+7.01%)	⬆️
src/io/ipc/read/array/boolean.rs	98.14% <0.00%> (+7.40%)	⬆️

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[jorgecarleitao]

Thanks a lot, @hohav !