Remove UseTerminalToGetActivatedEnvVars experiment

[kimadeline]

For #15530

Basically partially reverting d788d18

[karrtikr]

@kimadeline We have a work item to restore this experiment #13071, where we'll need the code that is being removed in this PR.

[kimadeline]

I don't think we'll reuse the code as-is, especially if we need to use a new API. The experiment will also need to be moved to Control Tower, instead of using the old experiments framework. Worst case, if we need this exact implementation, this PR can easily be reverted.

[karrtikr]

Makes sense 👍 We should probably link this PR in the issue though as this'll be very useful.

[kimadeline]

We should probably link this PR in the issue though as this'll be very useful.

You already did it when you mentioned it 😁

It is also now mentioned in #15530

[codecov-io]

Codecov Report

Merging #15538 (33b7e07) into main (75463af) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main   #15538    +/-   ##
=======================================
- Coverage     65%      64%    -1%     
=======================================
  Files        561      558     -3     
  Lines      27034    26870   -164     
  Branches    3924     3908    -16     
=======================================
- Hits       17590    17417   -173     
- Misses      8705     8717    +12     
+ Partials     739      736     -3     

Impacted Files	Coverage Δ
src/client/common/experiments/groups.ts	100% <ø> (ø)
src/client/interpreter/serviceRegistry.ts	100% <ø> (ø)
src/client/telemetry/index.ts	77% <ø> (ø)
src/client/common/experiments/manager.ts	85% <0%> (-11%)	⬇️
...rc/client/common/process/internal/scripts/index.ts	61% <0%> (-6%)	⬇️

[ericsnowcurrently]

LGTM