Don't use padding for horizontal stretchy extenders. (mathjax/MathJax#3429)

[dpvc]

This PR fixes a problem with horizontal stretchy character assemblies with middle pieces (like over- and under-braces) where the width is relatively small. The unneeded padding was causing the size to be incorrect when the total padding was larger than the width of the needed extender.

Resolves issue mathjax/MathJax#3429.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (afaf1ea) to head (91b1c2d).
⚠️ Report is 32 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/output/chtml/FontData.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1347      +/-   ##
===========================================
- Coverage    86.72%   86.71%   -0.01%     
===========================================
  Files          337      337              
  Lines        84145    84148       +3     
  Branches      3140     4769    +1629     
===========================================
  Hits         72971    72971              
+ Misses       11174    11154      -20     
- Partials         0       23      +23     

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[zorkow]

lgtm.