Drop legacy mat-select panel-width CSS in favor of MAT_SELECT_CONFIG

[rfontanarosa]

closes #1633

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.77%. Comparing base (2b5991b) to head (4093c11).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2510   +/-   ##
=======================================
  Coverage   60.77%   60.77%           
=======================================
  Files         116      116           
  Lines        2845     2845           
  Branches      428      428           
=======================================
  Hits         1729     1729           
  Misses       1055     1055           
  Partials       61       61           

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