fs_mgr: remove the repetitive and annoying bootconfig error print

The new changes to slotselect() call makes it try to find the
"slot" config before trying "slot_suffix" and that causes fs_mgr to
print the needless error and spam the kernel logs for no reason.
Remove the print.

Test: Builds for sailfish

Change-Id: I020575d70f4cd6e137e82c127b5d5234d709d0e3
Signed-off-by: Sandeep Patil <sspatil@google.com>