tests: fix spurious failure in 'insthook.test'

tests: fix spurious failure in 'insthook.test'

* tests/insthook.test (Makefile.am): Add a proper `uninstall-hook'
target to remove the symlink created by the `install-exec-hook'
target; this prevents "make distcheck" from failing spuriously.
Since we are at it, delete an extra blank line, and add a trailing
`:' command.