CLN: Remove unused cimports of float16

[jbrockmendel]

Part of the "move towards cythonize" sequence.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #18387 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18387      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         164      164              
  Lines       49718    49718              
==========================================
- Hits        45426    45417       -9     
- Misses       4292     4301       +9

Flag	Coverage Δ
#multiple	89.14% <ø> (ø)	⬆️
#single	39.61% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <0%> (-0.1%)	⬇️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #18387 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18387      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         164      164              
  Lines       49718    49718              
==========================================
- Hits        45426    45417       -9     
- Misses       4292     4301       +9

Flag	Coverage Δ
#multiple	89.14% <ø> (ø)	⬆️
#single	39.61% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <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
Powered by Codecov. Last update 1647a72...c1b20a1. Read the comment docs.

[codecov]

Codecov Report

Merging #18387 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18387      +/-   ##
==========================================
- Coverage   91.36%   91.34%   -0.02%     
==========================================
  Files         164      164              
  Lines       49718    49718              
==========================================
- Hits        45426    45417       -9     
- Misses       4292     4301       +9

Flag	Coverage Δ
#multiple	89.14% <ø> (ø)	⬆️
#single	39.61% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.8% <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
Powered by Codecov. Last update 1647a72...c1b20a1. Read the comment docs.

[jreback]

thanks!