[Router] remove dead modelselection config analyzer

config_analyzer.go lost its last caller in #1260 when the old training
flow was replaced by vsr validate. Since then every exported symbol in
the file (AnalyzeConfigForModelSelection, AnalyzeConfig,
ConfigAnalysisResult, MultiModelCategory, GetModelEndpoints) has zero
references in code, tests, or docs.

AnalyzeConfigForModelSelection was also broken for canonical configs:
it unmarshaled the file straight into RouterConfig, but canonical
configs keep decisions under routing: and the loader normalizes them,
so a bare unmarshal always saw 0 decisions. Rather than fix parse
logic nobody runs, delete the file.

This removes exported API from pkg/modelselection.

Signed-off-by: drivebyer <wuyangmuc@gmail.com>

Author: drivebyer