STYLE: Remove ITK version guards from Python examples

[hjmjohnson]

Remove the ITK version guards from ten Python examples. Examples require ITK 6,
so checks for ITK 4.7 through 5.2 can never fail.

What changes

Each example began with a check like:

if tuple(int(v) for v in itk.Version.GetITKVersion().split(".")) < (4, 9, 0):
    print("ITK 4.9.0 is required.")
    sys.exit(1)

The guard is removed, and so is import sys, which in every one of these files
was used only by the guard. 10 files, 40 lines deleted, nothing added.

Verification

Against ITK main with Python wrapping, all ten edited scripts import and run.

Five of the examples have Python tests that run in the full build, and all pass:
AddOffsetToIndex, StreamAPipeline, CreateACustomColormap,
SegmentBloodVessels, and SegmentBloodVesselsWithMultiScaleHessianBasedMeasure.

The other five are not exercised by ctest, for reasons unrelated to this change:

examples	why no Python test runs
ConvertAnitkImageTovtkImageData, ConvertAnRGBitkImageTovtkImageData	no Python test is registered
ApplyGradientRecursiveGaussian, ApplyGradientRecursiveGaussianWithVectorInput	ImageGradient is in Filtering_MODULES_IGNORED ("TODO: Fix ImageGradient examples and re-enable")
Perform2DTranslationRegistrationWithMeanSquares	gated behind ITKV4_COMPATIBILITY

pre-commit run --all-files passes.