lttng-tools: remove bogus comment

lttng-tools doesn't require bison or flex, it contains pre-generated
versions of the lexer and parser, so it can build perfectly fine
without bison and flex. We have removed both of these tools from our
mandatory dependencies since a bit of time, the Free Electrons
autobuilders don't have them installed, and the build succeeds.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>