fix: reduce through2 usage

[JustinBeckwith]

This is the part where I admit to not understanding how the other stream fix y'all landed works, or if this would be subject to the same kind of problem. I do know that the last 2 usages of through2 in the codebase cause sample test failures if you swap them out with PassThrough. Can I ask you both to poke holes in this?

[codecov]

Codecov Report

Merging #711 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
- Coverage   98.99%   98.99%   -0.01%     
==========================================
  Files          15       15              
  Lines       15311    15310       -1     
  Branches      932      932              
==========================================
- Hits        15157    15156       -1     
  Misses        151      151              
  Partials        3        3              

Impacted Files	Coverage Δ
src/index.ts	99.82% <100.00%> (-0.01%)	⬇️

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[crwilcox]

The default through.objseems equivalent to pass through. The other usages are nested streams to allow for retry. The other fix was due to one stream ending, but not unpiping causing write after end errors.
https://github.com/googleapis/nodejs-bigtable/blob/master/src/table.ts#L689

I think the instances in table.ts need to replace the usage with

stream.Transform(writableObjectMode: true, transform: (chunk, encoding, callback) => {
  // bespoke transform
});