[Cashtab] Improve alias error handling

Summary:
As per feedback in D15466, removing the redundant setIsValidAliasInput(false) calls upon server errors.

The remaining instances of setIsValidAliasInput(false) in handleAliasWalletChange, clearInputForms and handleAliasNameInput are untouched as they are genuine alias validation/availability logic.

Test Plan:

• npm test
• Register Alias button is enabled upon input of a valid and available alias, and registers all ok through the confirmation modal.
• Register Alias button is disabled upon input of a valid but unavailable alias

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15483