fix: remove unused env/config

[mshanemc]

What does this PR do?

remove the CODE_COVERAGE_REQUIREMENT env/config . Nothing is using it.

What issues does this PR fix or reference?

@W-14971005@

[iowillhoit]

From what I can tell this env var (and logic) was removed when this code was broken out of toolbelt

• Added here https://github.com/salesforcecli/toolbelt/commit/2d634b242530461061371c704c5ae2c5018440f2
  • June 8th, 2017
• Release here https://github.com/salesforcecli/toolbelt/commit/3225e8a94825bdc51335fb044e677f6e6ee383c2
  • June 23, 2017
• Tests added here https://github.com/salesforcecli/toolbelt/commit/1efc113ce02ccab73a7adf431f28090356c648f5
  • August 9th, 2017
• Removed from toolbelt here https://github.com/salesforcecli/toolbelt/commit/04c72dd086d03acfc8e0c726f422735ad32aa8c0
  • Mar 30, 2021

I cannot find any other reference to CODE_COVERAGE_REQUIREMENT or the apexTestApi file in our codebase. This env var used to exist in the HumanReporter test reporter. I cannot find any related logic (or env var) in the Human Reporter in plugin-apex or salesforcedx-apex

FYI: The epic this work was done in was [232] (b) Apex - Test:Run and Test:Report in the CLI & Open Source