refactor(styles): simplify the label's margin-bottom reset

[ahasall]

Since the following PR twbs/bootstrap#25561, there is a variable for setting the label's margin-bottom. We don't need to manually reset it.

[codecov-io]

Codecov Report

Merging #428 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   87.77%   87.77%           
=======================================
  Files          67       67           
  Lines         990      990           
  Branches       41       41           
=======================================
  Hits          869      869           
  Misses        103      103           
  Partials       18       18

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[tomastrajan]

@ahasall thanks for your contribution!