remove contour xDS server implementation

[skriss]

Closes #2134.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 80.70%. Comparing base (3cac9fc) to head (cef7058).

Files with missing lines	Patch %	Lines
cmd/contour/serve.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6568      +/-   ##
==========================================
- Coverage   81.05%   80.70%   -0.36%     
==========================================
  Files         133      130       -3     
  Lines       20026    19738     -288     
==========================================
- Hits        16232    15929     -303     
- Misses       3500     3519      +19     
+ Partials      294      290       -4     

Files with missing lines	Coverage Δ
cmd/contour/servecontext.go	84.34% <ø> (-0.18%)	⬇️
internal/contourconfig/contourconfiguration.go	98.17% <ø> (-0.02%)	⬇️
internal/xds/v3/callbacks.go	100.00% <100.00%> (ø)
internal/xdscache/v3/cluster.go	100.00% <ø> (ø)
internal/xdscache/v3/endpointslicetranslator.go	74.51% <ø> (-7.27%)	⬇️
internal/xdscache/v3/endpointstranslator.go	87.45% <ø> (-1.68%)	⬇️
internal/xdscache/v3/listener.go	91.54% <ø> (-0.45%)	⬇️
internal/xdscache/v3/route.go	96.90% <ø> (-0.56%)	⬇️
internal/xdscache/v3/runtime.go	97.43% <ø> (-2.57%)	⬇️
internal/xdscache/v3/secret.go	66.66% <ø> (-10.70%)	⬇️
... and 2 more

... and 2 files with indirect coverage changes

[skriss]

Calling this one ready for review, are others comfortable moving forward with this cleanup?