Revert "Compile System.Private.CoreLib with Crossgen2 by default (#44618)"

[stephentoub]

This reverts commit b79e4d3.

To test #45317.

[stephentoub]

/azp list

[azure-pipelines]

CI/CD Pipelines for this repository:

• coreclr-gc-longrunning
• coreclr-gc-simulator
• runtime-coreclr outerloop
• runtime-coreclr r2r-extra
• runtime-coreclr r2r
• runtime-coreclr runincontext
• runtime-coreclr crossgen2
• runtime-libraries-coreclr outerloop
• runtime-libraries-coreclr outerloop-windows
• runtime-libraries-coreclr outerloop-linux
• runtime-libraries-coreclr outerloop-osx
• runtime
• runtime-libraries enterprise-linux
• runtime-libraries stress-http
• runtime-libraries stress-ssl
• runtime-live-build
• runtime-coreclr crossgen2 outerloop
• coreclr-release-outerloop-nightly
• sync-runtime-to-mono
• dotnet-runtime-perf
• runtime-coreclr crossgen2-composite
• runtime-jit-experimental
• runtime-coreclr libraries-jitstress
• runtime-coreclr libraries-gcstress0x3-gcstress0xc
• runtime-coreclr libraries-gcstress-extra
• dotnet-linker-tests
• runtime-coreclr ilasm
• runtime-coreclr clrinterpreter
• runtime-coreclr crossgen2-composite gcstress
• runtime-libraries-mono outerloop
• runtime-staging
• runtime-coreclr superpmi
• runtime-coreclr jitrollingbuild

[trylek]

Thanks Steve! I've spent several hours thinking how to prove or disprove that the CG2 change is to blame but I guess you nailed down the most realistic option; after all, as I replied to you on that PR thread, that was one of the reasons why the change only modifies five characters in a single build script. I'm certainly prepared to immediately start investigating and fixing any potential CG2-related failures but initial probing of the behavior by means of a couple of instrumented lab jobs is definitely of great help.

[danmoseley]

Hmm I assume the intent was to merge this if the PR proved green - since the failures seemed fairly consistent. But perhaps the intention was to run CI several times to prove it?