[CASHTAB] Remove the .prettierignore file

Summary:
This file is used to exclude files from being overriden when prettier is
used with the --write option. It is not used to exclude files from
being parsed by the linter.
Since we use arcanist autofix feature to reformat the files, there is no
need for this file.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Subscribers: josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D8823