[filesystem] Silence undocumented exception.

[filesystem] Silence undocumented exception.

Filesystem function removeStorageStateChangeListener was throwing
NotFoundError exception if watchID argument did not correspond to
a registered listener. This commit removes this exception to match
documentation.

[Verification]

Exception is no longer thrown.
One TCT test case failed after this change, i removed it and commited
the change to tct repository. Review for TCT is available here:
https://review.tizen.org/gerrit/#/c/test/tct/web/api/+/196106/

Change-Id: I8cb7d99b5dda3f66f36c406c86330fb5d089287f
Signed-off-by: Michal Michalski <m.michalski2@partner.samsung.com>