Remove second caption for pander.summary.lm()

[krlmlr]

My use case was just caption = NULL, I'm not sure if the default caption still works with this PR.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@32e0f75). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #317   +/-   ##
=========================================
  Coverage          ?   79.57%           
=========================================
  Files             ?       12           
  Lines             ?     3555           
  Branches          ?        0           
=========================================
  Hits              ?     2829           
  Misses            ?      726           
  Partials          ?        0

Impacted Files	Coverage Δ
R/S3.R	83.86% <100%> (ø)

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