Remove obsolete PngTextProperties from png metadata

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

Removed obsolete PngTextProperties, TextData Property should be used instead.

[codecov]

Codecov Report

Merging #1189 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   82.53%   82.55%   +0.01%     
==========================================
  Files         692      692              
  Lines       29983    29976       -7     
  Branches     3390     3388       -2     
==========================================
  Hits        24746    24746              
+ Misses       4540     4533       -7     
  Partials      697      697              

Flag	Coverage Δ
#unittests	82.55% <ø> (+0.01%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Png/PngMetadata.cs	100.00% <ø> (+21.21%)	⬆️

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