Remove compat workaround for interceptors file path lookup.

[RikkiGibson]

Related to #71879
https://github.com/dotnet/roslyn/blob/main/docs/features/interceptors.md

Temporarily, for compatibility purposes, when the initial matching strategy outlined above fails to match any syntax trees, we will fall back to a "compat" matching strategy which works in the following way:

• A mapped path of each syntax tree is determined by applying /pathmap substitution to SyntaxTree.FilePath.
• For a given [InterceptsLocation] usage, the filePath argument value is compared to the mapped path of each syntax tree using ordinal string comparison. If exactly one syntax tree matches under this comparison, that is the referenced syntax tree. Otherwise, an error occurs.

Support for the "compat" strategy will be dropped prior to stable release.

Note: this issue is referenced in src/doc

[RikkiGibson]

Maybe we will end up using this issue to track entirely removing path-based interceptors (in favor of checksum-based.)

[jaredpar]

@RikkiGibson check to see if we still need this.

[RikkiGibson]

if we are removing this at the same time as filepath-line-col then close htis issue out and refer to it over there

[RikkiGibson]

The filepath-line-column scheme will be completely removed when support for .NET 8 is dropped, which means this compat workaround will also be removed at that time. #72855