Cleanup: configure.ac: remove redundant `AC_ARG_ENABLE` parameters

Cleanup: configure.ac: remove redundant `AC_ARG_ENABLE` parameters

According to the documentation [1], the last parameter of the
`AC_ARG_ENABLE()` macro (`action-if-not-given`) is executed only if
neither `--enable-foo` nor `--disable-foo` is provided.

So in cases where the feature is disabled by default, there is no need
to turn if off explicitly in the `action-if-not-given` parameter as
the macro will simply not set the `enable_foo` to `yes`.

Also, I fixed up the comment for the `enable_man_pages` variable.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.66/html_node/Package-Options.html

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I7416bed88ed1e719ef896f0ca0117b382d99f68f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3151
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>