Remove NUGET_EXPERIMENTAL_USE_NJ_FOR_FILE_PARSING

[zivkan]

NuGet Product(s) Affected

dotnet.exe, MSBuild.exe, NuGet.exe

Current Behavior

The environment variable NUGET_EXPERIMENTAL_USE_NJ_FOR_FILE_PARSING was added in NuGet 6.10, where the assets file parser was ported from Newtonsoft.Json to System.Text.Json. The environment variable was a fallback in case the STJ parser failed where the NJ parser did not, so customers could unblock themselves.

Desired Behavior

At the time this issue is being created, searching NuGet/Home issues for this environment variable does not find any results. This tells me there hasn't been a single case of a customer reporting a problem and this environment variable being discussed as a way to mitigate the error.

In other words, the STJ assets file parser doesn't appear to have blocking bugs.

Additional Context

Removing the NJ parser allows us to reduce a bunch of technical debt. In particular, as changes to the assets file are made in the future, it will not be necessary to implement (or provide a justification why not to) in both parsers.