[incfs] Remove the useless unmounting warning

It's pretty much given that unmounting will fail, as we don't
clear the page cache on before calling it. Much more interesting
to know if it ever happens from the first try

Bug: 267182195
Test: build + presubmits
Change-Id: I327c8ca95745e97456429c3c2fda22aeb685a12c
1 file changed