[Cashtab] Patch button css

Summary: we can't have "1px solid none" ... it "works" since we get "none" but it is bad syntax

Test Plan: npm test, no change in behavior expected, just cleaning up

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17624