fs_mgr: Remove incorrect free() on error paths in load_key()

This fixes a bug introduced by
https://android-review.googlesource.com/#/c/212781/ which would make
fs_mgr crash when hitting one of the error paths.

Bug: 28585197
Change-Id: I40e6612e2eb3e6f584e70c608afc6d4378d73c4f