Remove bytes encoder that was specifically for Django 1.11

[rsiemens]

Fixes #489

Seems this may have just been missed in #411

[codecov]

Codecov Report

Merging #490 (084c6da) into master (24aaa72) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
- Coverage   84.32%   84.29%   -0.03%     
==========================================
  Files           6        6              
  Lines         555      554       -1     
==========================================
- Hits          468      467       -1     
  Misses         87       87              

Impacted Files	Coverage Δ
MySQLdb/connections.py	81.11% <ø> (-0.14%)	⬇️

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[methane]

As I listed it in #472, I will remove it in v2.1.

But no schedule for v2.1 yet. Current master branch is for maintain v2.0. So I can not merge this PR soon.