Bug 1700106 - [devtools] Remove now-useless eslint files related to mochitests. r=jdescottes,Standard8

I kept a few having some overrides. But they may be irrelevant.
And I kept some eslint files for all folder that aren't matching the pattern matching "**/test*/**/browser*/".
Ideally we would rename these folder to match.

Last but not least, I identified one case where we were using mochitest file for xpcshell tests!

Differential Revision: https://phabricator.services.mozilla.com/D109481