Remove crate quickcheck_macros

[FreezyLemon]

This is the last dependency that requires version 1 of syn (see #3254):

$ cargo tree | grep -c "syn v1"
0

quickcheck_macros isn't actually required, and quickcheck is already on version 2 of syn. The code change that is needed to remove quickcheck_macros is very small, so I don't see a reason not to do it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (15f47f5) 88.48% compared to head (e9ee438) 88.48%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3328   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files          88       88           
  Lines       28230    28231    +1     
=======================================
+ Hits        24980    24981    +1     
  Misses       3250     3250           

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[shssoichiro]

Makes sense