checks: Flake8 F841 fixes in the wxpython directory part 1

[mshukuno]

Description

Flake8 F841 (local variable assigned to but never used) fixes in the gui/wxpython/animation and gui/wxpython/core directory.

Motivation and context

Changes require resolving a part of Flake8 warnings that are currently ignored. The issue title and number as follows.

• [Bug] Fix currently ignored Flake8 warnings #1522

How has this been tested?

animation:
Bify tested entering multiple raster datasets and it worked.

core:
Not tested.

Types of changes

Two declarations of unused functions were deleted.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as before)

Checklist

• PR title provides summary of the changes and starts with one of the
  pre-defined prefixes
• My code follows the code style
  of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[petrasovaa]

Looks like the .flake8 misses changes in wxpython/animation

[wenzeslaus]

This looks good to me. I was not sure about keeping reference to wx.App around, but I don't see any specific instructions in the doc. Plus now I see that it is a test. Let me know if you want more feedback from me here for some reason.

[mshukuno]

The initialization of wx.App is required by all wxPython applications, unless I misunderstood.
"Every wxPython app is an instance of wx.App."

My apologies for not understanding what you meant by "Plus now I see it's a test"?