xds: remove obsolete xDS transport custom dialer option

[yousukseung]

This PR removes #7586 which was made obsolete with #7997.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.15%. Comparing base (0003b4f) to head (35eeea2).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8079      +/-   ##
==========================================
- Coverage   82.32%   82.15%   -0.17%     
==========================================
  Files         387      387              
  Lines       39064    39062       -2     
==========================================
- Hits        32159    32092      -67     
- Misses       5593     5639      +46     
- Partials     1312     1331      +19     

Files with missing lines	Coverage Δ
internal/xds/bootstrap/bootstrap.go	65.72% <ø> (-0.20%)	⬇️

... and 23 files with indirect coverage changes