doc: remove unused definition in fs.md
Checklist
-
make -j4 test(UNIX), orvcbuild test(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc fs
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
make -j4 test (UNIX), or vcbuild test (Windows) passesdoc fs