chore: BigInt is always defined, do not conditionally check for it

[SimenB]

Summary

The global has existed since node 10.4 (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt#browser_compatibility), while the lowest version we support is node 10.13. So we don't need to conditionally look for it

Test plan

Green CI

[codecov-commenter]

Codecov Report

Merging #11979 (f571576) into main (7092dfb) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main   #11979   +/-   ##
=======================================
  Coverage   68.74%   68.74%           
=======================================
  Files         323      323           
  Lines       16649    16649           
  Branches     4808     4808           
=======================================
+ Hits        11445    11446    +1     
+ Misses       5171     5170    -1     
  Partials       33       33           

Impacted Files	Coverage Δ
packages/expect/src/asymmetricMatchers.ts	83.05% <ø> (ø)
packages/expect/src/utils.ts	96.37% <0.00%> (+0.51%)	⬆️

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[github-actions]

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