PERF: remove use of Panel & perf in rolling corr/cov

[jreback]

closes #17917

    before     after       ratio
  [aa9e0024] [872fe711]
-     1.70s    16.02ms      0.01  rolling.Pairwise.time_pairwise(None, 'corr', True)
-     1.84s    17.12ms      0.01  rolling.Pairwise.time_pairwise(10, 'corr', True)
-     1.84s    16.98ms      0.01  rolling.Pairwise.time_pairwise(1000, 'corr', True)
-     1.74s    15.59ms      0.01  rolling.Pairwise.time_pairwise(None, 'cov', True)
-     1.77s    15.63ms      0.01  rolling.Pairwise.time_pairwise(1000, 'cov', True)
-     1.83s    14.62ms      0.01  rolling.Pairwise.time_pairwise(10, 'cov', True)

[jreback]

cc @grumpyquant

if you'd give this a test drive. handling the MI is somewhat non-trivial here.

[codecov]

Codecov Report

Merging #19257 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19257      +/-   ##
==========================================
- Coverage   91.67%   91.65%   -0.02%     
==========================================
  Files         148      150       +2     
  Lines       48542    48722     +180     
==========================================
+ Hits        44501    44657     +156     
- Misses       4041     4065      +24

Flag	Coverage Δ
#multiple	90.02% <100%> (-0.02%)	⬇️
#single	41.74% <0%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
pandas/core/reshape/pivot.py	96.97% <100%> (+0.62%)	⬆️
pandas/core/window.py	96.32% <100%> (ø)	⬆️
pandas/core/indexes/accessors.py	90% <0%> (-0.2%)	⬇️
pandas/core/internals.py	95.46% <0%> (-0.01%)	⬇️
pandas/core/generic.py	95.91% <0%> (-0.01%)	⬇️
pandas/stats/api.py	100% <0%> (ø)
pandas/stats/moments.py	70.81% <0%> (ø)
pandas/core/frame.py	97.62% <0%> (+0.2%)	⬆️
pandas/core/indexes/multi.py	96.14% <0%> (+0.99%)	⬆️
pandas/plotting/_converter.py	66.95% <0%> (+1.73%)	⬆️

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