small improvements on the PorterDuff functions

[jongleur1983]

Explanation of the changes:

line 167:
dstW + srcW + blendW
= dstW + (source.W - blendW) + blendW
= dstW + source.W - blendW + blendW
= dstW + source.W

line 194:
dstW + blendW
= (destination.W - blendW) + blendW
= destination.W - blendW + blendW
= destination.W

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

removes two additions that are not necessary.

[codecov]

Codecov Report

Merging #1042 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1042   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files        1104     1104           
  Lines       48948    48948           
  Branches     3452     3452           
=======================================
  Hits        43987    43987           
  Misses       4250     4250           
  Partials      711      711

Impacted Files	Coverage Δ
.../PixelFormats/PixelBlenders/PorterDuffFunctions.cs	100% <100%> (ø)	⬆️

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