[cashtab-components] Remove all references to BCHA and replace with XEC

Summary: Related to task T2576. Removed all references to BCHA in cashtab-components and replaced them with XEC

Test Plan:
grep -r 'BCHA' src/ should return no results
grep -r 'BCHA' README.md should return no results.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11813