[LINTER] Improve lint-format-strings.sh performance

Summary:
The mapfile line originates from the initial script version which
worked on all files. It is no longer needed as arcanist passes each file
individually as an argument.

The performance has been mesured by running arc lint --everything with
this single linter enabled; the gain is about 20% on my machine.

Test Plan:

arc lint --everything

Should return no error

Comment one of the false positive at the beginning of the
lint-format-strings.py file.
Add a comment in the file from the commented false positive.

arc lint

The false positive should be catched by the linter.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2881