PR: Fix updated Spyder logos

[isabela-pf]

Description of Changes

Removes the upper left web in the corner of the last round of logos per request.

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes the last comment in #15829.

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct:
isabela-pf

[isabela-pf]

/show binder

[github-actions]

👈 Launch a Binder instance on this branch

[isabela-pf]

Here's how it looks in binder:

[isabela-pf]

• Add a background to spyder-readme-banner.png
• Move spyder-readme-banner.png to branding directory since it does not influence the application. This file will be deleted in this PR and added to the new path in PR: Add logos and guidelines outside the application #15980.

[steff456]

The latest commit works as expected for Mac Qt 5.9 and Qt 5.12. Thanks for this work @isabela-pf!