Remove unused wpcomSignup feature flag

[jaclync]

Closes: #7793

Description

Since the design has changed in favor of native account creation (signup) in #7891, I deleted the unused wpcomSignup feature flag and reverted the changes in #7794. Everything should work as before because the feature is disabled.

Testing instructions

You can do a confidence check by:

• Launch the app
• Log out and skip onboarding if needed
• Tap either login button
• When entering the WPCOM email, enter an email that isn't associated with a WPCOM account --> the login email error screen should be shown instead of continuing with the account creation flow in the WPAuthenticator library

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[itsmeichigo]

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8044-d399c0f on your iPhone

If you need access to App Center, please ask a maintainer to add you.