Remove <cat-toy> validation prompt from examples/textual_inversion/textual_inversion_sdxl.py

[mnoukhov]

The <cat-toy> validation prompt is a default choice for the example task in the README. But no other part of textual_inversion_sdxl.py references the cat toy and textual_inversion.py has a default validation prompt of None as well.

So bring textual_inversion_sdxl.py in line with textual_inversion.py and change default validation prompt to None

@jiqing-feng @sayakpaul

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[sayakpaul]

Thanks. Looks good to me.

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