ltrace: remove bogus comment in Config.in file

In commit dfaa18af ("ltrace: disable
on mips/mipsel"), ltrace was disabled on mips/mipsel due to build
issues, and a comment was added in the Config.in file to explain that
even though ltrace has mips/mipsel support, it isn't enabled because
it doesn't build.

Then, in commit d23cce19 ("ltrace:
enable for mips/mipsel"), the build of ltrace on mips/mipsel was
re-enabled, because it has been fixed upstream.

However, the comment in the Config.in comment was not removed in this
commit. Due to this, we have a comment that says "we don't allow
enabling ltrace on mips/mipsel" and the line right below precisely
allows to enable ltrace on mips/mipsel.

Fix this inconsistency by removing the no longer valid comment.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>