[Cashtab] Remove unused hook files

Summary:
I can't remember how or why these were placed. git blame has it before cashtab was in the monorepo.

grep -r useImage src and grep -r useInnerScroll src no output

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15818