[web] Stop ignoring fbjs in .flowconfig

Summary: Don't know why this was added initially, but we no longer have web/node_modules/fbjs and removing this doesn't cause any Flow errors.

Test Plan: Flow

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D960