CLN: Removing dead code in plotting and minor refactoring

[nrebena]

Follow up of #26185 .
This delete dead code from the MPLPlot._post_plot_logic_common and move the change from #26185 into LinePlot._post_plot_logic.

In detail, the change are only executed when the self.__need_to_set_index is True
The self._need_to_set_index variable is only set if you call _get_xticks.
The only place where _get_xticks is called is in LinePlot._make_plot.
Every children of LinePlot that redefine _make_plot also redefine _post_plot_logic.
Therefore I move the change in LinePlot._post_plot_logic

[codecov]

Codecov Report

Merging #26260 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26260      +/-   ##
==========================================
+ Coverage   91.97%   91.98%   +<.01%     
==========================================
  Files         175      175              
  Lines       52386    52379       -7     
==========================================
- Hits        48184    48179       -5     
+ Misses       4202     4200       -2

Flag	Coverage Δ
#multiple	90.53% <100%> (ø)	⬆️
#single	40.72% <0%> (-0.15%)	⬇️

Impacted Files	Coverage Δ
pandas/plotting/_core.py	83.91% <100%> (+0.27%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.71% <0%> (+0.1%)	⬆️

---

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[codecov]

Codecov Report

Merging #26260 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26260      +/-   ##
==========================================
+ Coverage   91.97%   91.98%   +<.01%     
==========================================
  Files         175      175              
  Lines       52386    52379       -7     
==========================================
- Hits        48184    48179       -5     
+ Misses       4202     4200       -2

Flag	Coverage Δ
#multiple	90.53% <100%> (ø)	⬆️
#single	40.72% <0%> (-0.15%)	⬇️

Impacted Files	Coverage Δ
pandas/plotting/_core.py	83.91% <100%> (+0.27%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	96.9% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.71% <0%> (+0.1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[jreback]

lgtm. @jorisvandenbossche or @TomAugspurger ?

[jreback]

thanks @nrebena