fs: remove superfluous type check in `_write()`

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

fs

Description of change

Type checking is already performed on the streams level, so this check could never actually fail.

CI: https://ci.nodejs.org/job/node-test-commit/6766/