xds: remove xds client fallback environment variable

[cjqzhao]

RELEASE NOTES:

• xds: remove support for GRPC_EXPERIMENTAL_XDS_FALLBACK environment variable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.31%. Comparing base (ac13172) to head (1573cbf).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8482      +/-   ##
==========================================
- Coverage   82.35%   82.31%   -0.05%     
==========================================
  Files         413      413              
  Lines       40518    40514       -4     
==========================================
- Hits        33368    33348      -20     
- Misses       5781     5794      +13     
- Partials     1369     1372       +3     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)
internal/xds/bootstrap/bootstrap.go	65.52% <ø> (-0.39%)	⬇️

... and 19 files with indirect coverage changes

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